avcodec/qtrle: call ff_reget_buffer() only when the picture data is going to change

avcodec/qtrle: call ff_reget_buffer() only when the picture data is going to change

ff_reget_buffer() will attempt to create a writable copy of the frame,
which is not needed when the decoder intends to return a reference to
the same buffer as the previous frame.

Should reduce data copy, hopefully achieving a similar speed up as
a9dacdeea6168787a142209bd19fdd74aefc9dd6 without dropping frames.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>