Allow our shaders to use samplerExternalOES, by declaring
that we use the relevant extension. Unfortunately, this
only works for gles, and requires different extensions for
gles2 and gles3. Yay
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.
