Catch some unhandled errors

- Trying to read pixels from a texture larger than the stage throws an
  exception in constrained mode.  Width/height are now clipped to stage
  width/height when reading pixels (the alternative would be to render
  each area separately and aggregate the pixel data, but this would
  probably not be performant).

- With multiple flambe instances on a page, an exception is thrown when
  trying to generate multiple AudioContexts.