In order to correctly report the error back to the user during the
creation of a scaled font, we need to support a nil object per error.
Instead of statically allocating all possible errors, lazily allocate
the nil object the first time we need to report a particular error.
This fixes the misreporting of an INVALID_MATRIX or NULL_POINTER that
are common user errors during the construction of a scaled font.
Partial revert of commit 0086db893c.
This is a follow to the earlier commit that allowed creation of scaled
fonts using a NULL font options (by interpreting the NULL as meaning
use the default options) to reflect the comments made by Behdad
(http://lists.cairographics.org/archives/cairo/2008-January/012714.html).
The intent is that the public font options getter/setter API has similar
defensive behaviour to that of the core objects - i.e. do not overwrite
the nil object and if the object is in error then return the default
value. For the indirect use of a NULL/nil font options (e.g. creation of
scaled fonts), then an error should be returned rather than crashing.
