Provide double width 4bpp output to lcd_draw_line

[ccawley2011]

By packing the palette into a single nibble and expanding it when the palette register as is, it's possible to output double-width 4bpp graphics data without much overhead. This is useful on RISC OS for rendering in modes 12 or 27, where the resulting buffer can be copied directly to the screen as-is.

This overlaps with the changes in the glfw_gles2 and sdl2-swrenderer branches, so it should probably be combined with one of them before merging.