After a short wild goose chase to see why
cairo_image_surface_fill_rectangles() was appearing in the profile,
tweak init_and_set_source_surface() to remove the redundant clear and
to propagate any errors in the auxiliary context.
The i915 is able to special case gradients with just 2 color stops to
avoid creating temporary gradient textures, so add a 3 stop linear
gradient to compare the speed difference.
Cover the similar source with min/mag scale factors as well, so we can
compare the performance impact with scaled image sources. This is useful
to distinguish between transport overhead and transform cost.
cairo_get_target() returns the original surface passed to
cairo_create(), and not the current destination as required when
testing drawing to the same surface using multiple contexts.
For completeness we also use the group target when creating similar
surfaces within the tests (to check that similar surfaces of similar
surfaces also work).
This will finally allow us to very easily add lots of other
tests that will similarly involve iterating over the various
sources and operators of interest.
