Add support for pagination in font files

[michielvandergeest]

When a font has multiple font styles (bold, italic, underline) or the number of characters is large (for example with hebrew or arabic text) multiple pngs need to be generated and pagination needs to be added in the font json.

The MSDF generator binary supports this behaviour, but it's not properly working in the MSDF generator. This needs to be added.

Note: possibly needs a change in the way that Blits utlizes the MSDF generator as well.