Issue 10378 (Part 1): Load 64bpp BMPs
This patch attempts to implement the first half of issue 10378, loading 64bpp BMPs.
Each RGB(A) channel uses 13 bits (2^13 max value), so it's scaled to fit our 16 bit integer image mode.
@Wormnest Hi! Feel free to try it out whenever you have time and let me know what you think. If you have any other 64bpp BMP sample images, I'd be happy to test those as well.