egui-wgpu: Support multisampling (#2878)

* Minimal implementation for egui wgpu multisampling support

* Update crates/egui-wgpu/CHANGELOG.md

Added pull request url

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Fixed cargo cranky error

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-wgpu/CHANGELOG.md

@@ -5,6 +5,7 @@ All notable changes to the `egui-wgpu` integration will be noted in this file.
 ## Unreleased
 * Add `read_screan_rgba` to the egui-wgpu `Painter`, to allow for capturing the current frame when using wgpu. Used in conjuction with `Frame::request_screenshot`. ([#2676](https://github.com/emilk/egui/pull/2676))
 * Improve performance of `update_buffers` ([#2820](https://github.com/emilk/egui/pull/2820))
+* Added support for multisampling (MSAA) ([#2878](https://github.com/emilk/egui/pull/2878))
 
 
 ## 0.21.0 - 2023-02-08

crates/egui-wgpu/src/winit.rs

@@ -81,6 +81,7 @@ pub struct Painter {
     support_transparent_backbuffer: bool,
     depth_format: Option<wgpu::TextureFormat>,
     depth_texture_view: Option<wgpu::TextureView>,
+    msaa_texture_view: Option<wgpu::TextureView>,
     screen_capture_state: Option<CaptureState>,
 
     instance: wgpu::Instance,
@@ -125,6 +126,7 @@ impl Painter {
             adapter: None,
             render_state: None,
             surface_state: None,
+            msaa_texture_view: None,
         }
     }
 
@@ -272,7 +274,7 @@ impl Painter {
                     height: size.height,
                     alpha_mode,
                 });
-                self.resize_and_generate_depth_texture_view(size.width, size.height);
+                self.resize_and_generate_depth_texture_view_and_msaa_view(size.width, size.height);
             }
             None => {
                 self.surface_state = None;
@@ -292,7 +294,7 @@ impl Painter {
             .map(|rs| rs.device.limits().max_texture_dimension_2d as usize)
     }
 
-    fn resize_and_generate_depth_texture_view(
+    fn resize_and_generate_depth_texture_view_and_msaa_view(
         &mut self,
         width_in_pixels: u32,
         height_in_pixels: u32,
@@ -325,11 +327,38 @@ impl Painter {
                 })
                 .create_view(&wgpu::TextureViewDescriptor::default())
         });
+
+        self.msaa_texture_view = (self.msaa_samples > 1)
+            .then_some(self.render_state.as_ref())
+            .flatten()
+            .map(|render_state| {
+                let texture_format = render_state.target_format;
+                render_state
+                    .device
+                    .create_texture(&wgpu::TextureDescriptor {
+                        label: Some("egui_msaa_texture"),
+                        size: wgpu::Extent3d {
+                            width: width_in_pixels,
+                            height: height_in_pixels,
+                            depth_or_array_layers: 1,
+                        },
+                        mip_level_count: 1,
+                        sample_count: self.msaa_samples,
+                        dimension: wgpu::TextureDimension::D2,
+                        format: texture_format,
+                        usage: wgpu::TextureUsages::RENDER_ATTACHMENT,
+                        view_formats: &[texture_format],
+                    })
+                    .create_view(&wgpu::TextureViewDescriptor::default())
+            });
     }
 
     pub fn on_window_resized(&mut self, width_in_pixels: u32, height_in_pixels: u32) {
         if self.surface_state.is_some() {
-            self.resize_and_generate_depth_texture_view(width_in_pixels, height_in_pixels);
+            self.resize_and_generate_depth_texture_view_and_msaa_view(
+                width_in_pixels,
+                height_in_pixels,
+            );
         } else {
             error!("Ignoring window resize notification with no surface created via Painter::set_window()");
         }
@@ -522,10 +551,18 @@ impl Painter {
                     .texture
                     .create_view(&wgpu::TextureViewDescriptor::default())
             };
+
+            let (view, resolve_target) = (self.msaa_samples > 1)
+                .then_some(self.msaa_texture_view.as_ref())
+                .flatten()
+                .map_or((&frame_view, None), |texture_view| {
+                    (texture_view, Some(&frame_view))
+                });
+
             let mut render_pass = encoder.begin_render_pass(&wgpu::RenderPassDescriptor {
                 color_attachments: &[Some(wgpu::RenderPassColorAttachment {
-                    view: &frame_view,
+                    view,
-                    resolve_target: None,
+                    resolve_target,
                     ops: wgpu::Operations {
                         load: wgpu::LoadOp::Clear(wgpu::Color {
                             r: clear_color[0] as f64,