swscale-test: allocate more memory to prevent scalers from writing out of bounds

Some converters (ie. unscaled rgb24 -> argb) may write some bytes out of
bounds. Ideally the converters should be fixed, but in the meantime we allocate
more memory to prevent heap corruption.

Originally committed as revision 31768 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale