* Add a "real" text API in addition to the current "toy" API.

* Implement text support for the image backend.

* Add arc support.

* Re-implement pattern support with a more PostScript-like API.

* Virtualize the backend interface so that the various backends can be
compiled conditionally.

* Verification, profiling, optimization.


Some notes on arc support
-------------------------

"Approximation of circular arcs by cubic poynomials", Michael Goldapp,
Computer Aided Geometric Design 8 (1991) 227-238.

To draw a unit arc from 0 to A with 0 < A < pi/2:

	Y
	
	|   .
	|  /  .
	| /    .
	|/A    .
	+------.-- X
	0      1

The deviation in radius is given by:

	rho(t) = sqrt ( x^2(t) + y^2(t) ) - 1

A simpler error function to work with is:

	e(t) = x^2(t) + y^2(t) - 1

And from Dokken[cite]: e(t) ~ 2 abs( rho(t) )

A single cubic Bezier spline approximation must have the 4 control points:

	(1, 0)
	(1, h)
	(cos(A) + h * sin(A), sin(A) - h * cos(A))
	(cos(A), sin(A))

Various approximations can be determined by selecting the value of
h. A convenient value, (though not optimal in terms of error), is:

	h = 4/3 * tan(A/4)

From which we can determine the maximum error:

	abs( max(e(t)) ) = 4/27 * (sin^6 (A/4)) / (cos^2 (A/4))
	    t in [0,1]
	
A comparison with PostScript
============================
Here's a list of several classes of PostScript operators indicating
which operators have equivalents in Cairo and which do not. If the Cairo

Oerators that are not yet in Cairo, but probably should be: arc,
arcto, strokepath, user path operators, rectclip?, clipsave/restore?,
setstrokeadjust?, currentdash, grestoreall?, initgraphics?,
currentgstate?, setgstate?, insideness testing?, showpage, copypage?,
erasepage?, setsmoothness?

Painting operators
------------------
in Cairo: stroke, fill, eofill (set_fill_rule/fill), image
(show_surface)

not in Cairo: erasepage, rectstroke, rectfill, shfill, colorimage,
imagemask

Path construction operators
---------------------------
in Cairo: newpath, moveto, rmoveto (rel_move_to), lineto, rlineto
(rel_line_to), curveto, rcurveto (rel_curve_to), closepath,
currentpoint, charpath (text_path)

not in Cairo: arc, arcn, arct, arcto, flattenpath, reversepath,
strokepath, clippath, pathbbox, pathforall

User path operators
-------------------
not in Cairo: ustroke, ufill, ueofill, uappend, upath, ustrokepath,
setbbox, ucache

Clipping
--------
in Cairo:  clip, eoclip (set_fill_rule/clip)

not in Cairo: initclip, rectclip, clipsave, cliprestore

Graphics state operators
------------------------
in Cairo: setlinewidth, currentlinewidth, setlinecap, currentlinecap,
setlinejoin, currentlinejoin, setmiterlimit, currentmiterlimit,
setdash

not in Cairo: setstrokeadjust, currentstrokeadjust, currentdash

Color specification operators
-----------------------------
in Cairo:  setrgbcolor, currentcolor

not in Cairo: setcolor, setgray, currentgray, currentrgbcolor,
sethsbcolor, currenthsbcolor, setcmykcolor, currentcmykcolor,
setcolorspace, currentcolorspace

Form and pattern operators
--------------------------
in Cairo: setpattern, makepattern (lock_pattern)

not in Cairo: execform

Whole-state manipulation
------------------------
in Cairo:  gsave (save), grestore (restore)

not in Cairo: grestoreall, initgraphics, gstate, currentgstate,
setgstate

Coordinate system and matrix operators
--------------------------------------
in Cairo: identmatrix (identity_matrix), initmatrix (default_matrix),
setmatrix, translate, scale, rotate, concatmatrix, currentmatrix,
transform (transform_point), dtransform (transform_distance)

not in Cairo: matrix, defaultmatrix, concat, itransform, idtransform,
invertmatrix

Insideness testing
------------------
not in Cairo: infill, instroke, inufill, inustroke, ineofill,
inueofill

Device setup
------------
not in Cairo: showpage, copypage, setpagedevice, currentpagedevice,
nulldevice

Glyph and font operators
------------------------
in Cairo: currentfont, definefont (font_create_for_ft_face),
undefine_font (font_destroy), findfont (font_create), makefont
(transform_font), setfont, scalefont, selectfont, show (show_text),
stringwidth (x/y in text_extents), xyshow (glyph_show -- but ignoring
current_point and using absolute positions)

not in Cairo, (and likely not needed): composefont, rootfont, ashow,
widthshow, awidthshow, xshow, xyshow, yshow, glyphshow, cshow, kshow,
FontDirectory, GlobalFontDirectory, StandardEncoding,
ISOLatin1Encoding, findencoding, setcachedevice, setcachedevice2,
setcharwidth

Graphics state operators (device-dependent)
-------------------------------------------
in Cairo:  setflat (set_tolerance), currentflat (current_tolerance)

not in Cairo: sethalftone, currenthalftone, setscreen, currentscreen,
setcolorscreen, currentcolorscreen, settransfer, currenttransfer,
setcolortransfer, currentcolortransfer, setblackgeneration,
currentblackgeneration, setundercolorremoval,
currentundercolorremoval, setcolorrendering, currentcolorrendering,
setoverprint, currentoverprint, setsmoothness, currentsmoothness

PostScript operators never to be in Cairo
-----------------------------------------
Operator Stack Manipulation Operators, Arithmetic and Math Operators,
Array Operators, Packed Array Operators, Dictionary Operators, String
Operators, Rational,Boolean,and Bitwise Operators, Control Operators,
Type,Attribute,and Conversion Operators, File Operators, Resource
Operators, Virtual Memory Operators, Miscellaneous Operators,
Interpreter Parameter Operators, Errors