GPU: Fix buffer overrun in Display Transfers

Display transfers with the horizontal downscaling flag were calculating
the wrong output size, causing them to write double the amount of data
intended. It is likely that this was perceived as correct due to a
separate bug in calculating source indices which caused the image to be
padded unless the previous bug was present.

This fixes both issues, correcting flickering issues in 3dscraft,
blargSnes and more (caused by the transfer overwriting the back buffer
which followed) as well as potentially fixing other crashes.