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.

eframe/src/web/backend.rs

@@ -170,6 +170,8 @@ impl AppRunner {
             storage: Some(&storage),
             #[cfg(feature = "glow")]
             gl: Some(painter.painter.gl().clone()),
+            #[cfg(feature = "wgpu")]
+            render_state: None,
         });
 
         let frame = epi::Frame {
@@ -178,6 +180,8 @@ impl AppRunner {
             storage: Some(Box::new(storage)),
             #[cfg(feature = "glow")]
             gl: Some(painter.gl().clone()),
+            #[cfg(feature = "wgpu")]
+            render_state: None,
         };
 
         let needs_repaint: std::sync::Arc<NeedRepaint> = Default::default();