Add a mipmap generation example.

[Kangz]

For example based on this code.

[austinEng]

I also should actually update things to use copyImageBitmapToTexture instead of the CPU readback and B2T copy. @kvark is copyImageBitmapToTexture implemented in Firefox or will I need to polyfill with writeTexture?

[kvark]

writeTexture is in, copyImageBitmapToTexture is not