Use gbm device from image copy protocol instead of default node

This fixes `BufferConstraints` errors for workspace capture on Nvidia
outputs of a Intel+Nvidia system. (That didn't happen on Amd+Nvidia;
maybe a linear stride alignment issue?)

src/backend/wayland/gbm_devices.rs

@@ -0,0 +1,43 @@
+use std::{
+    collections::hash_map::{self, HashMap},
+    fs, io,
+    os::unix::fs::MetadataExt,
+    path::{Path, PathBuf},
+};
+
+// TODO Purge gbm devices that are no longer needed/valid?
+#[derive(Default)]
+pub struct GbmDevices {
+    devices: HashMap<u64, (PathBuf, gbm::Device<fs::File>)>,
+}
+
+impl GbmDevices {
+    pub fn gbm_device(&mut self, dev: u64) -> io::Result<Option<(&Path, &gbm::Device<fs::File>)>> {
+        Ok(match self.devices.entry(dev) {
+            hash_map::Entry::Occupied(entry) => {
+                let (path, gbm) = entry.into_mut();
+                Some((path, gbm))
+            }
+            hash_map::Entry::Vacant(entry) => {
+                if let Some(value) = find_gbm_device(dev)? {
+                    let (path, gbm) = entry.insert(value);
+                    Some((path, gbm))
+                } else {
+                    None
+                }
+            }
+        })
+    }
+}
+
+fn find_gbm_device(dev: u64) -> io::Result<Option<(PathBuf, gbm::Device<fs::File>)>> {
+    for i in std::fs::read_dir("/dev/dri")? {
+        let i = i?;
+        if i.metadata()?.rdev() == dev {
+            let file = fs::File::options().read(true).write(true).open(i.path())?;
+            log::info!("Opened gbm main device '{}'", i.path().display());
+            return Ok(Some((i.path(), gbm::Device::new(file)?)));
+        }
+    }
+    Ok(None)
+}