Move sprite parsers into mod code

This unhardcodes our sprite parsers and moves them into the mod assemblies.  Mods now specify the formats that they use and the order in which files should be tested.

New sprite formats (such as gif) will now be much simpler to implement.  This also fixes the format-detection clash between R8 and TmpTS, which was blocking further progress on the TS tileset converter and ingame map editor.