[rtdef] Add BGR888 pixel format (#5765)

include/rtdef.h

@@ -1128,6 +1128,7 @@ enum
     RTGRAPHIC_PIXEL_FORMAT_BGR565 = RTGRAPHIC_PIXEL_FORMAT_RGB565P,
     RTGRAPHIC_PIXEL_FORMAT_RGB666,
     RTGRAPHIC_PIXEL_FORMAT_RGB888,
+    RTGRAPHIC_PIXEL_FORMAT_BGR888,
     RTGRAPHIC_PIXEL_FORMAT_ARGB888,
     RTGRAPHIC_PIXEL_FORMAT_ABGR888,
     RTGRAPHIC_PIXEL_FORMAT_RESERVED,