selmenu: use LRU cache so icons don't all need to be reloaded on scroll
uismall.bdf: set default character for missing glyphs
rendfont.cpp:
* encapsulate many BDF and BDC handling details
* make file I/O 64-bit clean, check for allocation errors
* more solid BDF parser with error messages and trace logging
* fix heap smash when building bitmaps for BDF fonts
* extend BDC format to support high planes and default character
* render default character if glyph not found for BDF/BDC