Fix BMP RLE delta escape reading from wrong file position

[bysiber]

The BMP RLE decoder's delta escape handler reads the displacement bytes twice from the file stream. It reads 2 bytes into bytes_read for validation, then calls self.fd.read(2) again instead of unpacking bytes_read:

bytes_read = self.fd.read(2)   # read delta bytes
if len(bytes_read) < 2:
    break
right, up = self.fd.read(2)    # reads 2 MORE bytes, discards the actual delta values

The real delta displacement values are thrown away, and right / up come from whatever happens to be next in the stream. Any RLE-compressed BMP that uses delta escape codes gets decoded with wrong pixel offsets, producing corrupted output.

[radarhere]

https://www.digicamsoft.com/bmp/bmp.html supports your change

2 Delta. The two bytes following the escape contain unsigned values indicating the horizontal and vertical offsets of the next pixel from the current position.

[bysiber]

Thanks for confirming with the spec, glad the fix aligns with what the BMP reference says.

[radarhere]

I've created bysiber#3 to add a test.