This commit adds a large number of pixel regressions to
pixel-test. All of these have at some point been failing in
blend-mode-test, and most of them do fail currently.
To be specific, with this commit, pixel-test reports 431 failed tests.
Support is added to pixel-test for verifying operations involving
masks. If a regression includes a mask, it is verified with the
pixel_checker API in in both unified and component alpha modes.
This test program contains a table of individual operator/pixel
combinations. For each pixel combination, images of various sizes are
filled with the pixels and then composited. The result is then
verified against the output of do_composite(). If the result doesn't
match, detailed error information is printed.
The initial 14 pixel combinations currently all fail.
