Fix upper mipmap boundary.
Fix upper mipmap boundary.

Correct maximum mipmap level is one less than number of mipmaps.

Fixes #788

aepsil0n/gfx-rs,tanderegg/gfx,tanderegg/gfx,ColinKinloch/gfx,sectopod/gfx-rs,davll/gfx,JohnColanduoni/gfx,ColinKinloch/gfx,JohnColanduoni/gfx,gfx-rs/gfx-rs,Bastacyclop/gfx