Caxton

• Replace the atlas implementation with a bucketed shelf allocation algorithm that wastes much less space than the previous one
• Add the /caxtondumpatlas command to dump the current Caxton glyph atlas for debugging
• Validate indices into pixel data associated glyphs when loading cached font data from the disk

Some caveats:

• Due to the new atlas implementation, this release imposes a hard limit of 255 on the range parameter in the *.ttf.json or *.otf.json file.

As usual, feel free to report bugs or performance regressions on the issue tracker.