Use the existing SpriteLoader cache for tilesets.
Use SpriteLoader for frame exporting.
Remove --transpose utility command.

The engine has supported transposed sprites for a long time now.
Add plumbing for mod-defined sprite loaders.
Move ShpTD sprite loading into Mods.Common.
Move ShpTS sprite loading into Mods.Common.
Move R8 sprite loading into Mods.D2k.
Move TmpRA sprite loading into Mods.Common.
Move TmpTD sprite loading into Mods.Common.
Move TmpTS sprite loading into Mods.TS.
Move ShpD2 sprite loading into Mods.Common.
Remove the remaining legacy sprite plumbing.
Support tmp(ts) templates with empty tiles.
Fix the legacy map editor.
Fix some bogus cell accesses in Map.
Merge pull request #6673 from pchote/unhardcode-sprites

Move sprite parsers into mod code