XCB/XVideo: add a TODO
swscale: remove constant parameter
Revert "swscale: scale the whole picture (fixes #12085)"

This reverts commit 44d4685b1ca4d1b58c4ce77d479103d482be5d14.
When using a scaling algorithm other than nearest neighbor, this change
caused the picture padding to be scaled with the video.
swscale: scale within the crop area, not top-left (really fixes #12085)

Unfortunately, this can cause a slight offset of the colour planes if the
top/left crop offset is not a multiple of the subsampling ratio.