Changed TilePalettePair to use indexed PixelFormats

Rewritten image conversion in TilePalettePair to generate Bitmaps with
indexed color PixelFormats, thus convering DS-side 4bpp and 8bpp images
to equivalent .NET-side Bitmaps, which ex. keep their original palette
when exporting; as .NET does not have a native 2bpp format, 2bpp images
(ex. the fonts) are converted to 8bpp