spicec: add sw canvas diff check

Setting CHECKDIFF=1 environment variable will compare the rendering of
the selected canvas with a software canvas. This is useful for debugging
some rendering issues, however it is far from being perfect, since it's
not able to tell whether one or the other is actually faulty. It's a
strong indication though of which operations are incorrect.

Ideally, all operations should be checked, however, a few of them are
disabled by default because they fail all the time, and it looks like
pixman is not very accurate for blending/compositing (at least visually,
it seems gl has better rendering)

0 files changed, 0 insertions, 0 deletions