This allows FreeTypeFontGenerator to create a BitmapFontData that renders glyphs as they are encountered so we can support fonts for languages that have many glyphs. See FreeTypeIncrementalTest.
The red boxes at the top are the glyph atlas pages, purposefully made tiny for testing. Better packing (a skyline variant) can be implemented in the future, for now it uses PixmapPacker as it used to.
I don't believe this breaks much, if anything, except that BitmapFont regions changes from TextureRegion to Array<TextureRegion> so adding a region on the fly is easier.
Related to #1589.
Added on the fly glyph rendering to FreeTypeFontGenerator.