Support for transparent backbuffer in wgpu winit binding (#2684)

* Support for transparent backbuffer in wgpu winit binding
Choose best fitting composite alpha mode on the fly.

* Compilation fix

* Add line to eframe CHANGELOG

* Attempt to mollify CI: try different way to install apt packages

---------

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

crates/eframe/CHANGELOG.md

@@ -13,6 +13,7 @@ NOTE: [`egui-winit`](../egui-winit/CHANGELOG.md), [`egui_glium`](../egui_glium/C
 * Fixed window position persistence for Windows ([#2583](https://github.com/emilk/egui/issues/2583)).
 * Update to `winit` 0.28, adding support for mac trackpad zoom ([#2654](https://github.com/emilk/egui/pull/2654)).
 * Fix bug where the cursor could get stuck using the wrong icon.
+* `NativeOptions::transparent` now works with the wgpu backend ([#2684](https://github.com/emilk/egui/pull/2684)).
 
 #### Web:
 * Prevent ctrl-P/cmd-P from opening the print dialog ([#2598](https://github.com/emilk/egui/pull/2598)).

crates/eframe/src/native/run.rs

@@ -958,6 +958,7 @@ mod wgpu_integration {
                     self.native_options.wgpu_options.clone(),
                     self.native_options.multisampling.max(1) as _,
                     self.native_options.depth_buffer,
+                    self.native_options.transparent,
                 );
                 pollster::block_on(painter.set_window(Some(&window)))?;
                 painter