Support loading json font atlas as generated by msdf-atlas-gen + fix font kerning offset to Float

[robinp]

Supports loading the output of Chlumsky/msdf-atlas-gen directly. This has the following benefits:

• existing forks of https://github.com/Yanrishatum/fontgen bitrot to the edge of compilability
• they only output to the BMFont format, which only has Int parameters for bounds/offsets, which is a real precision killer
• using their upstream tool directly (https://github.com/Chlumsky/msdf-atlas-gen) is much more straightforward, and retains precision

Example atlas generation (msdf):

/bin/msdf-atlas-gen -font a-font.ttf  -format png -pots -size 32 -json atlas.fnt -imageout atlas.png -yorigin top

Load it with channels=4, or specify -type sdf and use say channel 0.

Note: you might want to up the -size, depending on the font. Size=64 produced better spacings than size=32 for a random test font (arial-like, as is the sdf_font of the Text.hs example).

Edit: also, small change to take Float for glyph kerning. It was already stored as Float, but took Int (which sufficed so far). But with getting raw Float data now, it makes a real difference.

[robinp]

Random example - image diff on int-kern vs float-kern on a random test text, which started with "ATA...". The original A is diffed away completely since it matched, but the T and subsequent chars show a diff, due to the precision kerning. Rendered using HL target.