Added MGSpriteFontTextureProcessor.

This allows MG to compile images to SpriteFonts, using the params like
in XNA. This was just an adaption from the MGSpriteFontProcessor, so the
problems that MGSpriteFontProcessor has, MGSpriteFontTextureProcessor
will also have.
Merge branch 'develop' of https://github.com/mono/MonoGame into develop
Merge branch 'develop' of https://github.com/mono/MonoGame into develop
Merge branch 'develop' of https://github.com/mono/MonoGame into develop
Merge pull request #1616 from azchohfi/develop

Added MGSpriteFontTextureProcessor content processor.