[FEATURE REQUEST] Support for color indexed images (aka palettes, CLUT)

With color index to manage variations on an image would be piece of cake. Without need to create complicated materials and/or many unnecesary alphas. Maybe its not very usefull for 16bits color textures, but for the UI graphics where most of the time buttons use less than 256 colors would be very usefull. Also for the Paper2D option since its too complex for its own good.