Fix double init of Font.Image.

Generate was reloading the texture during construction.
Make texture loading nicer in Font.
Remove unused CharacterIndexPoints array from Font.
Split the font class into a FontBase and Font to allow creatubg custom fint implementations.

The base class introduction changes the internal API but hopefully impacts less consumers than renaming the public Font class. Also added a TextureFont to allow creating in-memory (generated) fonts.
Disable xml comment warnings in debug builds.
Allow setting CellPadding through the constructor for TextureFont.