Enable WebGPU backend in wgpu by default instead of WebGL

Instead, we expose a new `webgl` feature.

renderer/Cargo.toml

@@ -17,6 +17,7 @@ svg = ["iced_tiny_skia/svg", "iced_wgpu?/svg"]
 geometry = ["iced_graphics/geometry", "iced_tiny_skia/geometry", "iced_wgpu?/geometry"]
 tracing = ["iced_wgpu?/tracing"]
 web-colors = ["iced_wgpu?/web-colors"]
+webgl = ["iced_wgpu?/webgl"]
 
 [dependencies]
 raw-window-handle = "0.5"