Add `get_proc_address` in CreationContext (#4145)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to add commits to your PR.
* Remember to run `cargo fmt` and `cargo cranky`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

* Closes <https://github.com/emilk/egui/issues/4144>

crates/eframe/src/epi.rs

@@ -67,6 +67,10 @@ pub struct CreationContext<'s> {
     #[cfg(feature = "glow")]
     pub gl: Option<std::sync::Arc<glow::Context>>,
 
+    /// The `get_proc_address` wrapper of underlying GL context
+    #[cfg(feature = "glow")]
+    pub get_proc_address: Option<&'s dyn Fn(&std::ffi::CStr) -> *const std::ffi::c_void>,
+
     /// The underlying WGPU render state.
     ///
     /// Only available when compiling with the `wgpu` feature and using [`Renderer::Wgpu`].

crates/eframe/src/native/glow_integration.rs

@@ -284,12 +284,14 @@ impl GlowWinitApp {
             // Use latest raw_window_handle for eframe compatibility
             use raw_window_handle::{HasDisplayHandle as _, HasWindowHandle as _};
 
+            let get_proc_address = |addr: &_| glutin.get_proc_address(addr);
             let window = glutin.window(ViewportId::ROOT);
             let cc = CreationContext {
                 egui_ctx: integration.egui_ctx.clone(),
                 integration_info: integration.frame.info().clone(),
                 storage: integration.frame.storage(),
                 gl: Some(gl),
+                get_proc_address: Some(&get_proc_address),
                 #[cfg(feature = "wgpu")]
                 wgpu_render_state: None,
                 raw_display_handle: window.display_handle().map(|h| h.as_raw()),

crates/eframe/src/native/wgpu_integration.rs

@@ -262,6 +262,8 @@ impl WgpuWinitApp {
             storage: integration.frame.storage(),
             #[cfg(feature = "glow")]
             gl: None,
+            #[cfg(feature = "glow")]
+            get_proc_address: None,
             wgpu_render_state,
             raw_display_handle: window.display_handle().map(|h| h.as_raw()),
             raw_window_handle: window.window_handle().map(|h| h.as_raw()),

crates/eframe/src/web/app_runner.rs

@@ -76,6 +76,9 @@ impl AppRunner {
             #[cfg(feature = "glow")]
             gl: Some(painter.gl().clone()),
 
+            #[cfg(feature = "glow")]
+            get_proc_address: None,
+
             #[cfg(all(feature = "wgpu", not(feature = "glow")))]
             wgpu_render_state: painter.render_state(),
             #[cfg(all(feature = "wgpu", feature = "glow"))]