av_image_copy() accepts const uint8_t* const * as source;
lots of user have uint8_t* const * and therefore either
cast (the majority) or copy the array of pointers.

This commit changes this by adding a static inline wrapper
for av_image_copy() that casts between the two types
so that we do not need to add casts everywhere else.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>