This adds a few new functions to the JavaScript bindings. It also adds
our JavaScript docs into the CMake build system for machines that meet
the Python requirements.
While this solves the builder leak, it does not solve the tiny leak
incurred every time you call `getInstance` on a component manager
without calling embind's `delete` method afterwards. Since there's no
way to auto-delete component instances, this CL fixes up our sample
code and docstrings.
Fixes#429
Note that we need to allow clients to find out if compressed textures
are supported BEFORE the engine is created, so that they can start
downloading the correct set of texture files. To allow this, we now
expose a getSupportedFormats function that creates a throw-away canvas
in order to query extensions (creating a temporary canvas is a technique
that we also used in the old PNG decoder that we removed in 7b36ed4)
