Backports the text rendering code from 1.6 to 1.5.
Done by starting with the current master's engine/render/text_render
and backporting things as-needed until it worked.
CLX rendering and text input handling are also backported as part of
this change.
1. Do not modify the map after loading. Instead, return string views
(guaranteed to be null-terminated) from look up functions and return
the key directly if not found.
2. Use an `unorded_map` instead of `map` where available (C++20).
Saves a bit of RAM (~50 KiB) and improves lookup performance.
Gleb Mazovetskiy
obligaron
KPhoenix
qndel
Anders Jenbo