Mesa (main): gallium: implement compute pbo download

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Module: Mesa
Branch: main
Commit: e7b95619596167025d26f07efb04dc5bfd5e9aee
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=e7b95619596167025d26f07efb04dc5bfd5e9aee

Author: Mike Blumenkrantz <michael.blumenkrantz at gmail.com>
Date:   Thu Aug  5 15:28:52 2021 -0400

gallium: implement compute pbo download

this reworks PIPE_CAP_PREFER_BLIT_BASED_TEXTURE_TRANSFER into an
enum as PIPE_CAP_TEXTURE_TRANSFER_MODES, enabling drivers to choose
a (sometimes) faster, compute-based download mechanism based on a new
pipe_screen hook

compute pbo download is implemented using shaders with a prolog to convert
the input format to generic rgb float values, then an epilog to convert
to the output value. the prolog and epilog are determined based on a vec4
of packed ubo data which is dynamically updated based on the API usage

currently, the only known limitations are:
* GL_ARB_texture_cube_map_array is broken somehow (and disabled)
* AMD hardware somehow can't do depth readback?

otherwise it should work for every possible case

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer at amd.com>

Acked-by: Marek Olšák <marek.olsak at amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11984>

---

 docs/gallium/screen.rst                 |    4 +-
 src/gallium/include/pipe/p_defines.h    |    1 +
 src/mesa/meson.build                    |    1 +
 src/mesa/state_tracker/st_cb_texture.c  |   34 +-
 src/mesa/state_tracker/st_context.c     |    9 +-
 src/mesa/state_tracker/st_context.h     |    2 +
 src/mesa/state_tracker/st_pbo.c         |    9 +
 src/mesa/state_tracker/st_pbo.h         |    6 +
 src/mesa/state_tracker/st_pbo_compute.c | 1110 +++++++++++++++++++++++++++++++
 9 files changed, 1157 insertions(+), 19 deletions(-)

Diff:   http://cgit.freedesktop.org/mesa/mesa/diff/?id=e7b95619596167025d26f07efb04dc5bfd5e9aee