gallium: add PIPE_CAP_FBFETCH_ZS and expose extension

st/mesa will expose GL_ARM_shader_framebuffer_fetch_depth_stencil
if this new capability is supported by the driver.

Signed-off-by: Pavel Asyutchenko <sventeam@yandex.ru>
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13979>

docs/features.txt

@@ -316,6 +316,7 @@ Khronos, ARB, and OES extensions that are not part of any OpenGL or OpenGL ES ve
   GL_ARB_sparse_texture2                                DONE (radeonsi/gfx9+, zink)
   GL_ARB_sparse_texture_clamp                           DONE (radeonsi/gfx9+, zink)
   GL_ARB_texture_filter_minmax                          DONE (nvc0/gm200+, zink)
+  GL_ARM_shader_framebuffer_fetch_depth_stencil         DONE (llvmpipe)
   GL_EXT_color_buffer_half_float                        DONE (freedreno, i965, iris, llvmpipe, nv50, nvc0, radeonsi, zink)
   GL_EXT_depth_bounds_test                              DONE (i965/gen12+, nv50, nvc0, radeonsi, softpipe, zink)
   GL_EXT_memory_object                                  DONE (radeonsi, i965/gen7+, llvmpipe, zink)

docs/gallium/screen.rst

@@ -424,6 +424,8 @@ The integer capabilities:
   and a larger value would mean that multiple render targets are supported.
 * ``PIPE_CAP_FBFETCH_COHERENT``: Whether framebuffer fetches from the fragment
   shader can be guaranteed to be coherent with framebuffer writes.
+* ``PIPE_CAP_FBFETCH_ZS``: Whether fragment shader can fetch current values of
+  Z/S attachments. These fetches are always coherent with framebuffer writes.
 * ``PIPE_CAP_LEGACY_MATH_RULES``: Whether NIR shaders support the
   ``shader_info.use_legacy_math_rules`` flag (see documentation there), and
   TGSI shaders support the corresponding ``TGSI_PROPERTY_LEGACY_MATH_RULES``.