gallium: Add new cap PIPE_CAP_TEXTURE_BUFFER_SAMPLER

This indicates whether a driver wants samplers for buffer textures as
well as normal textures.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8300>

docs/gallium/screen.rst

@@ -165,6 +165,9 @@ The integer capabilities:
   TEXCOORD semantic.
   Also, TGSI_SEMANTIC_PCOORD becomes available, which labels a fragment shader
   input that will always be replaced with sprite coordinates.
+* ``PIPE_CAP_TEXTURE_BUFFER_SAMPLER``: Whether a sampler should still
+  be used for PIPE_BUFFER resources (normally a sampler is only used
+  if the texture target is PIPE_TEXTURE_*).
 * ``PIPE_CAP_PREFER_BLIT_BASED_TEXTURE_TRANSFER``: Whether it is preferable
   to use a blit to implement a texture transfer which needs format conversions
   and swizzling in gallium frontends. Generally, all hardware drivers with