TextureProcessor Fixes and Tests

This PR adds some fixes to the `TextureProcessor` as well as the start of some unit tests for it.

 - Fixed mipmap generation from occurring twice on compressed textures.
 - Fixed incorrect defaults in `TextureProcessor`.
 - `Texture2DContent` was not correctly initializing the `Faces`.
 - Fixed incorrect implicit operator in `MipmapChain`.
 - Implemented `overwriteExistingMipmaps` in `TextureContent.GenerateMipmaps`.