Add convenience functionality to Lua surface.mesh function

[Miles Bader]

• String loads a filename using load.mesh
  New arguments:
• Optional transform matrices (transform, separate load-transform, both defaulting to identity)
• Standard table-with-keywords argument style with keywords for:
  • material (sets mesh material)
  • file (loads mesh using load.mesh)
  • vertices (list of 3 * coordinates; list of positions too?)
  • normals (list of 3 * vector components; list of vectors too?)
  • uvs (list of 2 * UV coordinates)
  • triangles (list of 3 * vertex indices)
  • transform (applied to all of above)
  • file_transform (applied to file only, defaults to value of transform)
  • normal_indices ?? ... We don't support these natively, but can emulate them by duplicating vertices and adjusting references
  • uv_indices ?? ... same as normal_indices

This hopefully will make using surface.mesh convenient enough that we can drop the old utils.lua mesh-loading stuff...

See issue #37.