Some tests were using source surfaces created as ARGB32 even though all
alpha values are entirely opaque. This meant these tests were triggering
more fallbacks than strictly necessary for what the tests are ttrying to
do.
Port to use new cairo_create interface.
Rewrite all tests that were using cairo_set_target_surface to instead create a temporary cairo_t, (eventually to be replaced with cairo_begin_group).
Remove cairo_show_surface. Add new cairo_set_source_surface.
Remove _cairo_gstate_show_surface.
Replace calls to cairo_show_surface with cairo_set_source_surface; cairo_paint.
Fix messages to prefer - over _.
Fix documentation.
Three new tests to exercise set_source_surface more completely, (two of these are expected failures dues to outstanding bugs).
