[hal] Make DRM support optional.

In `wgpu_hal`, make support for `RawWindowHandle::Drm` and
`RawDisplayHandle::Drm` require that the `"drm"` feature be explicitly
requested, rather than supporting DRM handles by default.

The DRM API is for interacting with GPUs and displays directly via
system calls, without going through something like Mesa. Very few
`wgpu` users are driving hardware directly, and those that are are
sophisticated users, capable of finding and requesting the feature
themselves.

Supporting DRM in `wgpu_hal` adds dependencies on `drm` and its
supporting crates. The crates seem fine, but they do contribute to
download and build time.

Firefox definitely does not want to have to vendor `drm` and its
subcrates into its source tree, since Firefox should never be
interacting with graphics hardware directly.

Author: jimblandy

wgpu-hal/Cargo.toml

@@ -94,7 +94,6 @@ vulkan = [
     "dep:arrayvec",
     "dep:ash",
     "dep:bytemuck",
-    "dep:drm",
     "dep:gpu-descriptor",
     "dep:hashbrown",
     "dep:libc",

wgpu-hal/src/vulkan/drm.rs

@@ -1,4 +1,4 @@
-#![cfg(all(unix, not(target_vendor = "apple"), not(target_family = "wasm")))]
+#![cfg(feature = "drm")]
 
 use alloc::{string::ToString, vec::Vec};
 use core::{mem::MaybeUninit, num::NonZeroU32};

wgpu-hal/src/vulkan/instance.rs

@@ -264,11 +264,7 @@ impl super::Instance {
             extensions.push(ext::metal_surface::NAME);
             extensions.push(khr::portability_enumeration::NAME);
         }
-        if cfg!(all(
-            unix,
-            not(target_vendor = "apple"),
-            not(target_family = "wasm")
-        )) {
+        if cfg!(feature = "drm") {
             // VK_EXT_acquire_drm_display -> VK_EXT_direct_mode_display -> VK_KHR_display
             extensions.push(ext::acquire_drm_display::NAME);
             extensions.push(ext::direct_mode_display::NAME);
@@ -892,7 +888,7 @@ impl crate::Instance for super::Instance {
                 let connection = display.connection.expect("Pointer to X-Server is not set.");
                 self.create_surface_from_xcb(connection.as_ptr(), handle.window.get())
             }
-            #[cfg(all(unix, not(target_vendor = "apple"), not(target_family = "wasm")))]
+            #[cfg(feature="drm")]
             (Rwh::Drm(handle), Rdh::Drm(display)) => {
                 self.create_surface_from_drm_plane(display.fd, handle.plane)
             }