Factor out texture operations from Font to Sprite.

Allows us now to define arbitrary sprites using the apic quasi quoter in
conjunction with a PixelGen function that's responsible for turning
characters into RGBA colors.

However, we still need to properly convert the Word32 value into native
endian format, because right now 0xff000000 sets the _alpha_ channel.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>