clamp lower bound of mipmap width/height

[buhman]

Prior to this commit, width/height values could be right-shifted to zero (when loading non-square textures).

[SaschaWillems]

The tutorial does only use power-of-two textures. Not sure why this is required here.

[buhman]

It does not increase the complexity of the tutorial code significantly, and it also means that if someone wants to re-use the tutorial code with a different (non-square or non-power-of-two) texture, that the calculated mip level dimensions remain correct.