avcodec/msrledec: implement vertical offset in 4-bit RLE

The delta escape (2) is supposed to work the same in 4-bit RLE as in
8-bit RLE.  This is documented in the MSDN Bitmap Compression page:
https://msdn.microsoft.com/en-us/library/windows/desktop/dd183383(v=vs.85).aspx

The unchecked modification of line is safe, since the loop condition
(line >= 0) will check it before any pixel data is written.

Fixes ticket #5153 (output now matches ImageMagick for the provided sample).

Signed-off-by: Daniel Verkamp <daniel@drv.nu>
