Support arbitrary clip/transform stacks

[pcwalton]

The idea here is to have a compressed matrix/transform palette and clip out in the fragment shader. This is somewhat slow in that it could have a lot of unnecessary FS invocations, but it's on-GPU instead of on-CPU and doesn't mandate batch breaks unlike stencil-based solutions, and that seems like a win. The common cases are handled in the VS, so this should only happen in uncommon situations.

gw: pcwalton: my tentative plan was to add support to the shader to handle clips as either a simple rect (as now) or a compressed set of 4 clip plans with arbitrary transform. I haven't checked that this idea will actually work yet :) But yes, you're right - if it works, that could handle gradients.
gw: pcwalton: Yes, a palette like the rects. I think they could be compressed a lot in the uniforms too - since there is only one normal you need (two planes have the same normal, the other two are perpendicular). I think we could just specify one normal and 4 d values perhaps or something like that...

[pcwalton]

I think we won't need heterogeneous clip/transform stacks per se if we just have a list of clip quads (or parallelograms? maybe not in 3D) transformed into world space.

[kvark]

I suppose #498 covers that. Please feel free to re-open.