This is quite embarassing... it was returning the base instance instead of
the correct vao. No wonder that it was causing crash and at most drawing
issues.
This changes the drawing paradigm a bit. The VAO configuration is done
JIT-style and depends on context active shader.
This is to allow more flexibility for implementations to do optimization
at lower level.
The vao cache is now its own class to isolate the concept. It is this
class that is reference by the GLContext for ownership of the containing
VAO ids.
