[cairo-spans] Introduce a type to track which pixels combine in a compositing op.

A cairo_composite_rectangles_t contains the coordinates of rectangular
windows into each of the source pattern, mask, clip and destination
surface containing the pixels that will combine in a compositing
operation.  The idea is to have a uniform way to represent all the
translations involved rather than overloading parameters like src_x/y,
dst_x/y, etc., sometimes with different incompatible meanings across
functions.

src/cairo-rectangle.c

@@ -223,3 +223,24 @@ _cairo_box_contains_point (cairo_box_t *box, cairo_point_t *point)
 	return FALSE;
     return TRUE;
 }
+
+void
+_cairo_composite_rectangles_init(
+        cairo_composite_rectangles_t	*rects,
+        int				 all_x,
+        int				 all_y,
+        int				 width,
+        int				 height)
+{
+        rects->src.x = all_x;
+        rects->src.y = all_y;
+        rects->mask.x = all_x;
+        rects->mask.y = all_y;
+        rects->clip.x = all_x;
+        rects->clip.y = all_y;
+        rects->dst.x = all_x;
+        rects->dst.y = all_y;
+
+        rects->width = width;
+        rects->height = height;
+}

src/cairo-types-private.h

@@ -252,6 +252,30 @@ typedef struct _cairo_box_int {
     cairo_point_int_t p2;
 } cairo_box_int_t;
 
+
+/* Rectangles that take part in a composite operation.
+ *
+ * This defines four translations that define which pixels of the
+ * source pattern, mask, clip and destination surface take part in a
+ * general composite operation.  The idea is that the pixels at
+ *
+ *	(i,j)+(src.x, src.y) of the source,
+ *      (i,j)+(mask.x, mask.y) of the mask,
+ *      (i,j)+(clip.x, clip.y) of the clip and
+ *      (i,j)+(dst.x, dst.y) of the destination
+ *
+ * all combine together to form the result at (i,j)+(dst.x,dst.y),
+ * for i,j ranging in [0,width) and [0,height) respectively.
+ */
+typedef struct _cairo_composite_rectangles {
+        cairo_point_int_t src;
+        cairo_point_int_t mask;
+        cairo_point_int_t clip;
+        cairo_point_int_t dst;
+        int width;
+        int height;
+} cairo_composite_rectangles_t;
+
 typedef enum _cairo_direction {
     CAIRO_DIRECTION_FORWARD,
     CAIRO_DIRECTION_REVERSE

src/cairoint.h

@@ -263,6 +263,13 @@ _cairo_box_intersects_line_segment (cairo_box_t *box, cairo_line_t *line);
 cairo_private cairo_bool_t
 _cairo_box_contains_point (cairo_box_t *box, cairo_point_t *point);
 
+cairo_private void
+_cairo_composite_rectangles_init (cairo_composite_rectangles_t	*rects,
+                                  int				 all_x,
+                                  int				 all_y,
+                                  int				 width,
+                                  int				 height);
+
 /* cairo-array.c structures and functions */
 
 cairo_private void