Instead of having 3 different shaders for the different clipping
versions, just have one shader and use a preprocessor define to use
different clip functions.
That preprocessor define is set in the Makefile.
Also use foo.frag and foo.vert as the file extensions instead of using
foo.frag.glsl and foo.vert.glsl, as that's what glslc suggests as
extension.
Note: We interpolate premultiplied colors as per the CSS spec. This i
different from Cairo, which interpolates unpremultiplied.
So in testcases with translucent gradients, it's actually Cairo that is
wrong.
