In particular, fix the combination of luminance and alpha. We want to do
mask = luminance * alpha
and for inverted
mask = (1.0 - luminance) * alpha
so add a test that makes sure we do that and then fix the code and
existing tests to conform to it.
Our coverage computation only works for well-behaved
rects and rounded rects. But our modelview transform
might flip x or y around, causing things to fail.
Add functions to normalize rects and rounded rects,
and use it whenever we transform a rounded rect in GLSL.
Move the resources of each renderer to its subdirectory.
We've previously done that for the ngl renderer, but it
is better to be consistent and do it for all the renderers.
