Revert "swscale: fix integer overflows in RGB pixel writing."

This reverts commit 77d88b872d8cbb42738ede2d4fc098c16f204236.

The revert fixes actual overflows and a segfault as the variables
are signed and can be negative.

Conflicts:

	libswscale/swscale.c

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
