gallium: add BIND flags for R/W buffers and images

PIPE_CAPs and TGSI support will be added later. The TGSI support should be
straightforward. We only need to split TGSI_FILE_RESOURCE into TGSI_FILE_IMAGE
and TGSI_FILE_BUFFER, though duplicating all opcodes shouldn't be necessary.

The idea is:
* ARB_shader_image_load_store should use set_shader_images.
* ARB_shader_storage_buffer_object should use set_shader_buffers(slots 0..M-1)
  if M shader storage buffers are supported.
* ARB_shader_atomic_counters should use set_shader_buffers(slots M..N)
  if N-M+1 atomic counter buffers are supported.

PIPE_CAPs can describe various constraints for early DX11 hardware.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/docs/source/screen.rst

@@ -430,8 +430,10 @@ resources might be created and handled quite differently.
   process.
 * ``PIPE_BIND_GLOBAL``: A buffer that can be mapped into the global
   address space of a compute program.
-* ``PIPE_BIND_SHADER_RESOURCE``: A buffer or texture that can be
-  bound to the graphics pipeline as a shader resource.
+* ``PIPE_BIND_SHADER_BUFFER``: A buffer without a format that can be bound
+  to a shader and can be used with load, store, and atomic instructions.
+* ``PIPE_BIND_SHADER_IMAGE``: A buffer or texture with a format that can be
+  bound to a shader and can be used with load, store, and atomic instructions.
 * ``PIPE_BIND_COMPUTE_RESOURCE``: A buffer or texture that can be
   bound to the compute program as a shader resource.
 * ``PIPE_BIND_COMMAND_ARGS_BUFFER``: A buffer that may be sourced by the