Make the map height&width arrow buttons snap to POT values

The arrow buttons next to the resolutions increment the width/height by one, which is not really useful. I guess this the default behavior of a number input control. Anyway, it would make a lot more sense if they would change the resolution to next/previous power of 2. User could still type whatever manually to the value (although non-POT resolutions should not be used in textures anyway). 
