Faster large map display; texture-related fixes

• Faster and more interactive map load, especially when using full-screen or zooming out via "View | Zoom out further", and faster model export by reducing file accesses. Memory size for map caching is now selectable, under "Help | Map memory". Multithreading added for map loading, similar to Minutor. (Thanks to Karel Koudelka for getting the ball rolling on this.)
• Make bed, bell, and turtle eggs textures not be considered cutouts, so that they render better in Eevee and elsewhere.
• Fixed ChannelMixer and TileMaker to properly find the barrier, decorated_pot_side, and shulker box sides and bottoms. Derive end_portal more the way that Minecraft makes it. Other fixes, such as removing incorrect warnings. TileMaker updated to 3.32, ChannelMixer to 1.25.
• Track non-graphical attributes for .schem export and import. Things like fence gates and trapdoors and mob heads being powered, ages of kelp, various vines, mangrove propagule, and many others.
• Remade custom texture terrain packs, fixing a few textures.
• Documentation cleanup and link fixing.