Ok, I've bitten the bullet, and am explicitly calculating image data

min/max in the Image class. I am doing this in an attempt to eliminate
duplicate min/max calculations in the ImageTexture class, and to remove
the 'sampled min/max' bug, where the display/clipping ranges are set to
those of a sample, instead of those of the full image. Unfortunately,
load time for big images is now a necessity. Bugfix in Selection -
handling an empty selection.