Enable iced wgpu backend, which seems to perform better

A good software renderer probably should perform fine where subsurfaces
are used like for the actual images. Perhaps more improvement is needed
in the renderer.

Cargo.lock

@@ -2767,6 +2767,7 @@ dependencies = [
  "iced_sctk",
  "iced_style",
  "iced_tiny_skia",
+ "iced_wgpu",
  "iced_widget",
  "lazy_static",
  "palette",

Cargo.toml

@@ -33,6 +33,10 @@ calloop-wayland-source = "0.3.0"
 version = "0.14.1"
 features = ["fluent-system", "desktop-requester"]
 
+[features]
+default = ["wgpu"]
+wgpu = ["libcosmic/wgpu"]
+
 [profile.dev]
 # Not usable at opt-level 0, at least with software renderer
 opt-level = 1