Unlike avcodec_default_get_buffer2(), this version does not allocate more than
what the lavu image helper functions consider should be allocated for a given
frame.
Since the get_buffer2() documentation does not require any kind of buffer
padding for any of the planes, this should help detect bugs in our DR1 decoders
if they read beyond the end of the buffer, simulating what some library users
might experience when they use their own custom get_buffer2() implementations.
Signed-off-by: James Almer <jamrial@gmail.com>