Work on graphics and bindings

Working on drawing multiple scene_objects within a scene. Need to work
on draw parameters buffers, since GL_shader_draw_parameters might not
always be supported (and is not, in my machine's case), so we can
have a proper offset into the transforms buffer of a scene_object's
instances.

Clean up some bindings code to send proper type error messages and
conform to Lua idioms regarding optional/missing parameters