Scene renderer: Clear the internal material reference when the
connection is deleted.
Added new camera preset: Orbit camera. A simple graph
re-implementation of the polar camera model from the MV client
prototype.
Utils: Added sync attribute to tag for dynamically loaded scripts.
Added webgl debug context script. It's always loaded, even when
not in active use, but the overhead is minimal (< 1000 lines).
Renderer changes:

* Always set UV addressing mode to wrap.
* Add (commented out) code to use the webgl context debug wrapper.
* Add support for per-map UV scale and offset. This is currently
  done in the pixel shader, but should be moved to the vertex
  shader now that we have support for per-map UV-channel index.
* Various redundancies in the code to automatically compute
  shaders removed or reduced.

Exporter changes:

* Support for per-map UV-channel index, UV scale + offset as set
  in the relevant texture.
* Improved error handling, including emitted warnings about
  missing UV channel references in individual texture slots.
Renderer: Better shader error handling and logging.