It's a bit involved as we need to ensure that gl.Context.current doesn't
outline the Application instance, so we need to:
- remember the Application object when it gets constructed (and clear
it again when it gets destructed)
- in gl.Context.current check if there's an active Application (which
means sharing data across two different Python modules, and even
though pybind11 docs suggest to "simply export a symbol", this
*cannot* possibly work in practice; instead we share data using a
Python capsule), and increase its refcount when returning the Context
instance
- decrease the Application refcount again when the Context gets
destructed
Only the double ones, exposed as floats, because the extra ALU required
by doubles is negligible to function call overhead. It'll be different
for non-scalar types, but here I use this.
Vladimír Vondruš