egui-wgpu: turn off the default features of wgpu (emilk#3875)

This makes all `wgpu` features opt-in for `egui-wgpu` users.

For `eframe`, I opted to enable some `wgpu` features so users can still
use `eframe` without having to also opt-in to extra ewgpu features
(eframe is batteries-included).

Cargo.toml

@@ -53,7 +53,7 @@ glow = "0.13"
 puffin = "0.18"
 raw-window-handle = "0.6.0"
 thiserror = "1.0.37"
-wgpu = { version = "0.19.1", features = [
+wgpu = { version = "0.19.1", default-features = false, features = [
     # Make the renderer `Sync` even on wasm32, because it makes the code simpler:
     "fragile-send-sync-non-atomic-wasm",
 ] }

crates/eframe/Cargo.toml

@@ -168,7 +168,12 @@ pollster = { version = "0.3", optional = true } # needed for wgpu
 glutin = { version = "0.31", optional = true }
 glutin-winit = { version = "0.4", optional = true }
 puffin = { workspace = true, optional = true }
-wgpu = { workspace = true, optional = true }
+wgpu = { workspace = true, optional = true, features = [
+  # Let's enable some backends so that users can use `eframe` out-of-the-box
+  # without having to explicitly opt-in to backends
+  "metal",
+  "webgpu",
+] }
 
 # mac:
 [target.'cfg(any(target_os = "macos"))'.dependencies]

crates/egui-wgpu/Cargo.toml

@@ -45,7 +45,7 @@ bytemuck = "1.7"
 log = { version = "0.4", features = ["std"] }
 thiserror.workspace = true
 type-map = "0.5.0"
-wgpu.workspace = true
+wgpu = { workspace = true, features = ["wgsl"] }
 
 #! ### Optional dependencies
 ## Enable this when generating docs.