Gash - Better Bitmaps

I wrote the part of the new NextGenGraphics constructor that carves the xGAGRAPH file apart into chunks. Since decompression doesn't happen immediately, the raw compressed chunks have to be labeled with their size, taking into account the fact that some special chunks have their decompressed size hard-coded (rather than it being the first four bytes of the raw chunk).

I also realized that my existing SixteenColorBitmap class, had I tried to use it here, would have been both poorly named and really difficult to deal with. It doesn't support masking (you need MaskedSixteenColorBitmap for that) and doesn't have a very convenient way of dealing with palettes. So, I wrote a LowColorBitmap class that reduces the craziness (using the .NET Color struct instead of 4-tuples) and has built-in support for masking.