Fix panic on wgpu GL backend due to new screenshot capability (#3078)

* Triage for GL backend

* And cargo-fmt

* Changelog update with PR and issue

* Update crates/eframe/src/epi/mod.rs

Co-authored-by: Andreas Reich <r_andreas2@web.de>

* Update crates/egui-wgpu/src/winit.rs

Co-authored-by: Andreas Reich <r_andreas2@web.de>

* Add "supports_screenshot" to surface state

* Cranky fix

* fmt

---------

Co-authored-by: Andreas Reich <r_andreas2@web.de>

crates/eframe/src/epi/mod.rs

@@ -799,6 +799,7 @@ impl Frame {
     /// * Called in [`App::update`]
     /// * [`Frame::request_screenshot`] wasn't called on this frame during [`App::update`]
     /// * The rendering backend doesn't support this feature (yet). Currently implemented for wgpu and glow, but not with wasm as target.
+    /// * Wgpu's GL target is active (not yet supported)
     /// * Retrieving the data was unsuccessful in some way.
     ///
     /// See also [`egui::ColorImage::region`]

crates/egui-wgpu/CHANGELOG.md

@@ -3,6 +3,7 @@ All notable changes to the `egui-wgpu` integration will be noted in this file.
 
 
 ## Unreleased
+* Fix panic on wgpu GL backend due to new screenshot capability ([#3068](https://github.com/emilk/egui/issues/3068), [#3078](https://github.com/emilk/egui/pull/3078)
 
 
 ## 0.22.0 - 2023-05-23

crates/egui-wgpu/src/winit.rs

@@ -7,6 +7,7 @@ struct SurfaceState {
     alpha_mode: wgpu::CompositeAlphaMode,
     width: u32,
     height: u32,
+    supports_screenshot: bool,
 }
 
 /// A texture and a buffer for reading the rendered frame back to the cpu.
@@ -136,10 +137,15 @@ impl Painter {
         render_state: &RenderState,
         present_mode: wgpu::PresentMode,
     ) {
+        let usage = if surface_state.supports_screenshot {
+            wgpu::TextureUsages::RENDER_ATTACHMENT | wgpu::TextureUsages::COPY_DST
+        } else {
+            wgpu::TextureUsages::RENDER_ATTACHMENT
+        };
         surface_state.surface.configure(
             &render_state.device,
             &wgpu::SurfaceConfiguration {
-                usage: wgpu::TextureUsages::RENDER_ATTACHMENT | wgpu::TextureUsages::COPY_DST,
+                usage,
                 format: render_state.target_format,
                 width: surface_state.width,
                 height: surface_state.height,
@@ -218,12 +224,16 @@ impl Painter {
                     wgpu::CompositeAlphaMode::Auto
                 };
 
+                let supports_screenshot =
+                    !matches!(render_state.adapter.get_info().backend, wgpu::Backend::Gl);
+
                 let size = window.inner_size();
                 self.surface_state = Some(SurfaceState {
                     surface,
                     width: size.width,
                     height: size.height,
                     alpha_mode,
+                    supports_screenshot,
                 });
                 self.resize_and_generate_depth_texture_view_and_msaa_view(size.width, size.height);
             }
@@ -485,6 +495,15 @@ impl Painter {
             )
         };
 
+        let capture = match (capture, surface_state.supports_screenshot) {
+            (false, _) => false,
+            (true, true) => true,
+            (true, false) => {
+                log::error!("The active render surface doesn't support taking screenshots.");
+                false
+            }
+        };
+
         {
             let renderer = render_state.renderer.read();
             let frame_view = if capture {
@@ -566,7 +585,7 @@ impl Painter {
         } else {
             None
         };
-        // Redraw egui
+
         {
             crate::profile_scope!("present");
             output_frame.present();