gallium: add PIPE_CAP_CLEAR_TEXTURE and clear_texture prototype

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/gallium/docs/source/context.rst

@@ -227,6 +227,10 @@ is is also possible to only clear one or the other part). While it is only
 possible to clear one surface at a time (which can include several layers),
 this surface need not be bound to the framebuffer.
 
+``clear_texture`` clears a non-PIPE_BUFFER resource's specified level
+and bounding box with a clear value provided in that resource's native
+format.
+
 ``clear_buffer`` clears a PIPE_BUFFER resource with the specified clear value
 (which may be multiple bytes in length). Logically this is a memset with a
 multi-byte element value starting at offset bytes from resource start, going

src/gallium/docs/source/screen.rst

@@ -281,6 +281,8 @@ The integer capabilities:
 * ``PIPE_CAP_COPY_BETWEEN_COMPRESSED_AND_PLAIN_FORMATS``:
   Whether copying between compressed and plain formats is supported where
   a compressed block is copied to/from a plain pixel of the same size.
+* ``PIPE_CAP_CLEAR_TEXTURE``: Whether `clear_texture` will be
+  available in contexts.
 
 
 .. _pipe_capf: