Update to glow 0.12

[Rob2309]

This PR intends to resolve #686.

Instead of using the std::mem::transmute workaround mentioned in several issues (e.g. grovesNL/glow#187 (comment)), I resorted to using a typesafe solution until a proper mechanism is put into place.

The way I recreate the GL objects from the raw u32 handles is to directly create new glow::Native* objects, since their inner field is public. This at least ensures that any breaking layout changes will be detected, which is not necessarily the case in the transmute() workaround.

Of course, this makes compiling for wasm completely impossible right now, but as far as I can tell, this is not possible anyway.

[Rob2309]

I just dug into the wasm types a bit and it should probably be possible to call get_parameter() instead of get_parameter_i32(). In WebGL get_parameter returns the corresponding WebGl* objects which can by cast via dyn_into(). While requiring special code only for wasm, this would probably work.

I could test this but I don't know yet how to compile an imgui app for wasm or whether its really possible atm.

[Rob2309]

Update: with the current glow it actually seems impossible to use the values returned from get_parameter(...) as they return the actual GL objects but the glow bind_* calls expect a key into a map of GL objects.

[dbr]

Great, thanks for this!