gui: wgpu fallback renderer for desktop (#31)

Reviewed-on: https://code.gri.mw/GUI/grim/pulls/31

Cargo.lock

@@ -2485,6 +2485,7 @@ dependencies = [
  "objc2-foundation 0.2.2",
  "parking_lot 0.12.5",
  "percent-encoding",
+ "pollster",
  "profiling",
  "raw-window-handle",
  "static_assertions",
@@ -2492,6 +2493,7 @@ dependencies = [
  "wasm-bindgen-futures",
  "web-sys",
  "web-time",
+ "wgpu",
  "windows-sys 0.61.2",
  "winit",
 ]