Add WebGL2 Adapter::new_external() + context accessor

CHANGELOG.md

@@ -105,6 +105,7 @@ By @andyleiserson in [#9321](https://github.com/gfx-rs/wgpu/pull/9321).
 #### GLES
 
 - Added support for GLSL passthrough. By @inner-daemons in [#9064](https://github.com/gfx-rs/wgpu/pull/9064).
+- Implement `Adapter::new_external()` for WebGL2 (just like EGL/WGL) to import an external WebGL2 rendering context, and expose the imported context back through `Adapter::adapter_context()` / `Device::context()`. By @pepperoni505 in [#9438](https://github.com/gfx-rs/wgpu/pull/9438).
 
 #### DX12
 

wgpu-hal/src/gles/web.rs

@@ -179,6 +179,44 @@ impl crate::Instance for Instance {
     }
 }
 
+impl super::Adapter {
+    /// Creates a new external adapter from an existing WebGL2 rendering context.
+    ///
+    /// # Safety
+    ///
+    /// - The underlying WebGL2 context must be valid (not lost).
+    /// - The underlying WebGL2 context must be valid when interfacing with any objects returned by
+    ///   wgpu-hal from this adapter.
+    /// - The underlying WebGL2 context must be valid when dropping this adapter and when
+    ///   dropping any objects returned from this adapter.
+    pub unsafe fn new_external(
+        webgl2_context: web_sys::WebGl2RenderingContext,
+        options: wgt::GlBackendOptions,
+    ) -> Option<crate::ExposedAdapter<super::Api>> {
+        let glow_context = glow::Context::from_webgl2_context(webgl2_context.clone());
+        unsafe {
+            Self::expose(
+                AdapterContext {
+                    glow_context,
+                    webgl2_context,
+                },
+                options,
+            )
+        }
+    }
+
+    pub fn adapter_context(&self) -> &AdapterContext {
+        &self.shared.context
+    }
+}
+
+impl super::Device {
+    /// Returns the underlying WebGL2 `AdapterContext`.
+    pub fn context(&self) -> &AdapterContext {
+        &self.shared.context
+    }
+}
+
 #[derive(Debug)]
 pub struct Surface {
     canvas: Canvas,