lavfi/drawutils: overhaul to improve pixel format support

- No longer mixes u8 and u16 component accesses (this was UB)
- De-duplicated 8->16 conversion
- De-duplicated component -> plane+offset conversion
- De-duplicated planar + packed RGB
- No longer calls ff_fill_rgba_map
- Removed redundant comp_mask data member
- RGB0 and related formats no longer write an alpha value to the 0 byte
- Non-planar YA formats now work correctly
- High-bit-depth semi-planar YUV now works correctly

libavfilter/drawutils.h

@@ -37,7 +37,6 @@ typedef struct FFDrawContext {
     enum AVPixelFormat format;
     unsigned nb_planes;
     int pixelstep[MAX_PLANES]; /*< offset between pixels */
-    uint8_t comp_mask[MAX_PLANES]; /*< bitmask of used non-alpha components */
     uint8_t hsub[MAX_PLANES];  /*< horizontal subsampling */
     uint8_t vsub[MAX_PLANES];  /*< vertical subsampling */
     uint8_t hsub_max;