Transform feedback

This code was originally based on the Panfrost implementation, but has been
improved in a number of ways.

1. Transform feedback programs are dispatched generically with Gallium calls,
   rather than emitting something hardware-specific. This is cleaner and
   portable to future GPUs.

2. Transform feedback with indexed draws is now fixed, by lowering to an index
   buffer pull.

3. Transform feedback with buffer overflows is now fixed, by correctly
   bounds checking in transform feedback programs.

4. Transform feedback with strips/fans/loops are fixed, by correctly
   tessellating to the underlying primitives as required by OpenGL.

5. Transform feedback with QUADS is fixed, by tessellating to triangles as
   required by OpenGL.

That said, the code is still not in its final form.

1. It still does not support indirect draws. This will require a substantial
   overhaul to do tracking on the GPU instead of the CPU. Currently we force
   unroll indirect draws (slow but kosher in GL, treif in Vulkan). This isn't
   hard to solve but I'm not going to duplicate the code until the algorithms
   are otherwise complete because it's a lot easier to hack on the CPU versions
   than the GPU versions.

2. It still does not support primitive restart. This has especially nasty
   interactions with transform feedback. Again we force unroll to non-primitive
   restart forms, again slow but kosher in GL but treif in Vulkan. This is a lot
   harder to deal with. I sketched out something really nasty in my notebook
   (hinging on efficient GPU prefix sums) but I'm not in a hurry to type this
   out.

3. There will be interactions with geometry and tessellation shaders and I don't
   think I can get the core code here future-proofed without actually bringing
   up the new shader stages.

As such, this is a hard fork of the panfrost code for now, I'm not trying to
share the code (although it *would* clear out almost all of panfrost's transform
feedback related piglit failures).

Passes dEQP-GLES3.functional.transform_feedback.* and most of the relevant
piglits.

Depends on mesa/mesa!22123 (closed)