Fix saving/loading glyph 255 in RegisterFont

The glyph for character 255 (lower case y with two dots above it) was
rendered, but it's glyph information was not stored in fontInfo_t and
not saved into .dat file (including the ones in Team Arena).

Attempting to load it from existing .dat font files is fine because
shader name is "" and gets 0 handle. The handle was already 0 anyway.
Fix negative glyph index in Team Arena text functions

Team Arena's text functions cast signed char values to int and use as an array index.
This works fine for values 0 to 127, but not for -128 to -1 which are a negative array index.
Instead use "character & 255" like client and original Q3 ui/cgame string drawing code.