Add CommonMCOBJ export, minor fixes

• Mineways implementation of CommonMCOBJ, a standardized header for OBJ file data.
• Fix error message when failing to load a schematic.
• Tilemaker: slightly more accurate heightfield to normal texture conversion formula. Fix TGA reader for when the input and output number of channels don't match. Added some more error messages to handle bad PNGs. Now at version 3.23.
• JG-RTX texture pack update, in particular getting some fixed normals textures.