Add basic multipass support.

Add basic multipass support.

Now we can render all the passes defined in a shader file, and we even
get the texture coordinates of each pass the right way round. As part of
this. I added a Texture class that groups the texture ID, width and
height into a single class, so _render_pass() can return all the
information about the new texture instead of passing tuples around
everywhere.

Still need to handle the special-casing the spec calls an "implicit
pass".