Move the OpenCl object cast from the generated entry points to the
stubs and front end, to make it possible to properly use static_cast.
This removes the limitation that the front end objects have to be
standard layout (and makes it possible to use virtual functions),
which is consistent with other front end objects.

Move the back end initialization from the stubs to the entry point
functions, which fixes a bug where the back ends were not initialized
during validation.

Move more code from the stubs to the front end, to keep the stubs
light.

Remove unused function `default_return_value` from
`generate_entry_points.py`.

Bug: angleproject:5904
Change-Id: Id999ad6c537888017bf3252c6f6e088b7d4c7984
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2897245
Commit-Queue: John Plate <jplate@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>