Refactor adding of vertices

[seanlynch]

• Changed can_build to return True
• Factored out -1 bone index check (still treats it as unsigned)
• Factored out adding each vertex to the surface
• Add vertices without indices and then let SurfaceTool generate indices

Tested on Windows with some other changes to get it to compile that I haven't included. Appears to be exactly as broken as the original, but it should be a bit faster and use less GPU memory since it only adds each vertex once per index and then reindexes instead of adding all vertexes to each surface.

[fire]

Superseded by: https://github.com/V-Sekai/godot-pmx/pull/2