improve color creation macros
improve color creation macros

- make sure colors are cast to SwfdecColor to prevent signedness warnings
- add SWFDEC_COLOR_OPAQUE(color) to "remove" the alpha component
- add SWFDEC_COLOR_WHITE to specify a white color

freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec,mltframework/swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec