Re-implement PaintCallbacks With Support for WGPU (#1684)

* Re-implement PaintCallbacks With Support for WGPU

This makes breaking changes to the PaintCallback system, but makes it
flexible enough to support both the WGPU and glow backends with custom
rendering.

Also adds a WGPU equivalent to the glow demo for custom painting.

egui_glow/src/lib.rs

@@ -9,7 +9,7 @@
 
 pub mod painter;
 pub use glow;
-pub use painter::Painter;
+pub use painter::{CallbackFn, Painter};
 mod misc_util;
 mod post_process;
 mod shader_version;

egui_glow/src/painter.rs

@@ -4,7 +4,7 @@ use std::{collections::HashMap, sync::Arc};
 
 use egui::{
     emath::Rect,
-    epaint::{Color32, Mesh, Primitive, Vertex},
+    epaint::{Color32, Mesh, PaintCallbackInfo, Primitive, Vertex},
 };
 use glow::HasContext as _;
 use memoffset::offset_of;
@@ -68,6 +68,29 @@ pub struct Painter {
     destroyed: bool,
 }
 
+/// A callback function that can be used to compose an [`egui::PaintCallback`] for custom rendering
+/// with [`glow`].
+///
+/// The callback is passed, the [`egui::PaintCallbackInfo`] and the [`Painter`] which can be used to
+/// access the OpenGL context.
+///
+/// # Example
+///
+/// See the [custom3d_glow] demo source for a detailed usage example.
+///
+/// [custom3d_glow]:
+///     https://github.com/emilk/egui/blob/master/egui_demo_app/src/apps/custom3d_wgpu.rs
+pub struct CallbackFn {
+    f: Box<dyn Fn(PaintCallbackInfo, &Painter) + Sync + Send>,
+}
+
+impl CallbackFn {
+    pub fn new<F: Fn(PaintCallbackInfo, &Painter) + Sync + Send + 'static>(callback: F) -> Self {
+        let f = Box::new(callback);
+        CallbackFn { f }
+    }
+}
+
 impl Painter {
     /// Create painter.
     ///
@@ -381,7 +404,11 @@ impl Painter {
                             screen_size_px,
                         };
 
-                        callback.call(&info, self);
+                        if let Some(callback) = callback.callback.downcast_ref::<CallbackFn>() {
+                            (callback.f)(info, self);
+                        } else {
+                            tracing::warn!("Warning: Unsupported render callback. Expected egui_glow::CallbackFn");
+                        }
 
                         check_for_gl_error!(&self.gl, "callback");