Fix custom font fallback rendering

[gta66666668]

Fixes custom text rendering for glyphs that are not present in the selected Meteor font, such as Chinese characters.

The previous custom font renderer pre-packed a fixed set of Latin, Greek, Cyrillic, and symbol ranges. Characters outside those ranges could not render even when the OS had a suitable font installed.

Changes:

• load an available system fallback font for common CJK font families
• let Font use multiple font buffers
• lazily pack missing glyphs into the texture atlas
• upload pending glyphs before GUI/HUD custom text rendering
• handle Unicode code points instead of only Java char values

Validation:

• ./gradlew.bat build --stacktrace passes

This PR is based on master and does not include the Minecraft 26.2 update branch.