Support Indexed8 Alphamap format for SafePicture and glyphs

Rationale: both glyphs and SafePictures are alpha maps that don't store the colour in the original data and they both needs colouring during blit. Now the glyphs are their own case and we need to hack a special case for SafePicture as it is now in ARGB8888 format as all bitmaps. This wastes memory (8-bit vs. 32-bits per pixel) and generates unnecessary code paths in the runtime.

As a side effect, one should consider creating a compression method for Indexed8, perhaps RLE8, which could be pretty straightforward to adapt from RLE32.