Expose glow::Context via epi::Frame

[emilk]

Since we are converging on using glow as the rendering backend for eframe on both native and web, it would be quite useful to expose the glow GL context to the user, in particular in order to do paint-to-texture from within eframe.

This would need to be opt-in thing though (optional dependency of epi) and be fallible (e.g. frame.glow_context() should return None if we aren't using a glow renderer).

This would obviously be for advanced users.

I'm not sure if there is interest in this though. My thinking is that if one wants to do 3D stuff with egui, one should probably use bevy_egui, egui-macrquad, or similar. Still, eframe has many features these libraries lack (reactive mode, better i/o on web).

[coderedart]

there could be some considerations for the still imaginary multi-window feature. gl contexts are usually per window.
I don't see any negatives here. as long as it is marked unsafe, those who want to experiment can do so. some more questions:
does the user get to request a specific version of opengl (4.6?) at the start of program?
what state is okay to change (since its all global) eg: drawing into texture means changing framebuffers/attachments and maybe more exotic stuff like polygon mode /depth test / stencil test which egui doesn't set every frame right now.

there could be a slight chance that people will be hitting discord with a lot of opengl questions and we might need to pin renderdoc download link at the top

[emilk]

I'm now looking into adding a custom Shape with a callback to which the glow context will be passed. That would allow users to embed 3D stuff and other custom painting directly into an egui Ui. This will be similar to how Dear ImGui has AddCallback/UserCallback