Increase bitmap font character size (currently 256 px)

The current maximum font size for bitmap images is 256px by 256px. This is very limiting as bitmap fonts are the primary way resource packs can display images on a player's screen, often used for HUDs, GUIs, etc. Due to most player's screen resolution being larger than 256px, this severely limits the use and requires workarounds such as splitting up the image into multiple font glyphs.