Add option to initialize on existing wgpu setup (#5319)

When mixing and matching eframe with other wgpu applications
(https://github.com/tracel-ai/burn in my case), it can be helpful to use
an existing wgpu setup to initialize eframe with. This PR changes the
WpuConfiguration (in a non-backwards compat way :/), to either take some
options how to create a wgpu setup, or an existing wgpu setup
(consisting of an instance, adapter, device and queue).

* [x] I have followed the instructions in the PR template

---------

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

crates/egui-wgpu/src/winit.rs

@@ -87,7 +87,7 @@ pub struct Painter {
     depth_format: Option<wgpu::TextureFormat>,
     screen_capture_state: Option<CaptureState>,
 
-    instance: wgpu::Instance,
+    instance: Arc<wgpu::Instance>,
     render_state: Option<RenderState>,
 
     // Per viewport/window:
@@ -116,10 +116,15 @@ impl Painter {
         support_transparent_backbuffer: bool,
         dithering: bool,
     ) -> Self {
-        let instance = wgpu::Instance::new(wgpu::InstanceDescriptor {
-            backends: configuration.supported_backends,
-            ..Default::default()
-        });
+        let instance = match &configuration.wgpu_setup {
+            crate::WgpuSetup::CreateNew {
+                supported_backends, ..
+            } => Arc::new(wgpu::Instance::new(wgpu::InstanceDescriptor {
+                backends: *supported_backends,
+                ..Default::default()
+            })),
+            crate::WgpuSetup::Existing { instance, .. } => instance.clone(),
+        };
 
         Self {
             configuration,