sws: Fix:
libswscale/swscale_unscaled.c:805:5: warning: passing argument 1 of ‘check_image_pointers’ from incompatible pointer type
libswscale/swscale_unscaled.c:774:12: note: expected ‘uint8_t **’ but argument is of type ‘const uint8_t * const*’
libswscale/swscale_unscaled.c:809:5: warning: passing argument 1 of ‘check_image_pointers’ discards qualifiers from pointer target type
libswscale/swscale_unscaled.c:774:12: note: expected ‘uint8_t **’ but argument is of type ‘uint8_t * const*’

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
