swscale: fix invalid conversions and memory problems.

Fixes problems where rgbToRgbWrapper() is called even though it doesn't
support this particular conversion (e.g. converting from RGB444 to
anything). Thirdly, fixes issues where rgbToRgbWrapper() is called for
non-native endiannness conversions (e.g. RGB555BE on a LE system).
Fourthly, fixes crashes when converting from e.g. monowhite to
monowhite, which calls planarCopyWrapper() and overwrites/reads because
n_bytes != n_pixels.