panfrost: Implement transform feedback

Midgard has no hardware support for transform feedback, so we simulate
it in software. Lucky us.

What Midgard does do is write out vertex shader outputs to main memory
unconditonally. Fragment shaders read varyings back from main memory;
there's no on-chip storage for varyings. Whether this was a reasonable
design is a question I will not be engaging in this commit message.

What that does mean is that, in some sense, Midgard *always* does
transform feedback uncondtionally, and there's no way to turn off
transform feedback. Normally, we would allocate some scratch memory
every frame to store the varyings in an arbitrary format (interleaved
for simplicity), and then feed that scratch to the fragment shader and
discard when the rendering completes.

The only difference now is that sometimes, for some buffers, we use a BO
provided to us by Gallium and a format provided by Gallium, instead of
allocating the memory and choosing the format ourselves. This has some
limitations -- in particular, it only works at vec4 granularity, so a
corresponding GLSL linkage patch is needed to correctly implement
transform feedback for non-vec4 types. Nevertheless, given the hardware
already works in this admittedly-bizarre fashion, transform feedback is
"free". Or, at least, it's no more expensive than any other rendering.

Specifically not implemented is dynamically-sized transform feedback
(i.e. with geometry/tesselation shaders).

Spoiler alert: Midgard has no support for geometry *or* tessellation
shaders, despite advertising support. They get compiled to *massive*
compute shaders. How's that for checkbox compliance?

Signed-off-by: Alyssa Rosenzweig <[email protected]>
Reviewed-by: Boris Brezillon <[email protected]>

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