* io-bmp.c (grow_buffer): New function to avoid crashes
	on unchecked reallocs.
	(DecodeHeader, DecodeColormap, decode_bitmasks,
	DoCompressed): Use grow_buffer instead of g_realloc
	throughout. Change signatures where necessary to pass the
	errors up.
	(OneLine16): Fix loading of 16bpp BI_RGB bmps.
	(DoCompressed): Rewritten to properly support BI_RLE4 and
	skips and jumps.