eframe does not expose &mut egui_glow::Painter

**Is your feature request related to a problem? Please describe.**
`eframe::Frame` under wgpu provides `wgpu_render_state()`, which allows us to mutably access the underlying `egui_wgpu::Renderer` and call e.g. `register_native_texture`. However, under glow, the `egui_glow::Painter` can only be accessed by way of a paint callback, and only immutably behind a `&Painter`.

**Describe the solution you'd like**
`eframe::Frame` should expose a `glow_render_state` that allows mutable access to the `Painter` in addition to the `gl` context.

**Describe alternatives you've considered**
I considered hacking my way around a paint callback until I realized that I still couldn't call `register_native_texture`.

Start by locating eframe::Frame's existing wgpu_render_state() API and the glow paint-callback path described in the issue. Trace how the egui_glow::Painter and gl context are stored and exposed, then determine the API and lifecycle changes needed for mutable access. Done means Frame exposes a glow_render_state with mutable Painter access alongside the gl context, with coverage for registering native textures.

Written by the indexing model from the issue text.