Add calls to wl_shm_buffer_begin/end_access

This wraps all accesses to an SHM buffer between wl_shm_buffer_begin
and end so that wayland-shm can install a handler for SIGBUS and catch
attempts to pass the compositor a buffer that is too small.
diff --git a/src/rpi-renderer.c b/src/rpi-renderer.c
index 8fb562d..1940db7 100644
--- a/src/rpi-renderer.c
+++ b/src/rpi-renderer.c
@@ -342,6 +342,8 @@
 		pixman_region32_intersect(&write_region,
 					  &write_region, region);
 
+	wl_shm_buffer_begin_access(buffer->shm_buffer);
+
 #ifdef HAVE_RESOURCE_WRITE_DATA_RECT
 	/* XXX: Can this do a format conversion, so that scanout does not have to? */
 	r = pixman_region32_rectangles(&write_region, &n);
@@ -376,6 +378,8 @@
 	    width, r->y2 - r->y1, 0, r->y1, ret);
 #endif
 
+	wl_shm_buffer_end_access(buffer->shm_buffer);
+
 	pixman_region32_fini(&write_region);
 
 	return ret ? -1 : 0;