gallium: add interface to clear buffers

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

src/gallium/docs/source/context.rst

@@ -218,6 +218,11 @@ 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_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
+for size bytes. It is guaranteed that size % clear_value_size == 0.
+
 
 Drawing
 ^^^^^^^