gallium: add pipe cap for scissored clears and pass scissor state to clear() hook

this adds a new pipe cap that drivers can support which enables passing buffer
clears with scissor test enabled through to be handled by the driver instead
of having mesa draw a quad

also adjust all existing clear() hooks to have the new parameter

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4310>

src/gallium/drivers/r300/r300_blit.c

@@ -202,6 +202,7 @@ DEBUG_GET_ONCE_BOOL_OPTION(hyperz, "RADEON_HYPERZ", FALSE)
 /* Clear currently bound buffers. */
 static void r300_clear(struct pipe_context* pipe,
                        unsigned buffers,
+                       const struct pipe_scissor_state *scissor_state,
                        const union pipe_color_union *color,
                        double depth,
                        unsigned stencil)