Kis Tangent Normal Paintop Now Works in 16-Bit and 32-Bit Image Modes.

This updates the normal paint-top to work properly in non-8bit image modes. I believe that most
normal maps are usually 8bit, but this will at least prevent a crash and produce the expected
behavior regardless of image mode.
BUG: 398826

(cherry picked from commit f7e40ac3)