#40 Add grayscaler to paletted (8-bit) images

[Daniel Mealha Cabrita]

ConvertToGrayscale supports only 24/32-bit images.
It may also apply to palette images with 8-bit/color (>16 colors) converting the pictures to 8-bit grayscale and supressing the raster buffer.
PNG supports lossless compression of that.

The saving consists of ommiting the CLUT (palette) data (from ~51 to ~768 bytes depending on how many colors were used).