host/x11: default to XDamage capture; drop --untimed from viewers

X11 full-desktop capture used `ximagesrc use-damage=false`, which copies
the whole root window every frame. On servers without working MIT-SHM
(and CPU-bound everywhere else) this collapses to ~1 fps — a field test
over an xlibre host played back at roughly one frame per minute. Default
to `use-damage=true` (XDamage re-grabs only changed regions); keep
`PIXELPASS_X11_NO_DAMAGE=1` as an escape hatch for driver artifacts.

Also drop `--untimed` from both mpv invocations (viewer banner + the
interactive launcher). `--untimed` displays each frame as it decodes and
ignores audio timestamps, which drifts a shared *video* progressively
out of sync with its audio. Pacing to the audio clock keeps A/V synced
at a negligible latency cost.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/host/x11.rs

@@ -37,12 +37,22 @@ pub async fn start(opts: &HostOpts, quality: &EffectiveQuality) -> Result<Captur
         }
     };
 
+    // XDamage capture (`use-damage=true`) only re-grabs changed screen
+    // regions instead of copying the whole root window every frame. On a busy
+    // desktop that is the difference between a usable framerate and ~1 fps —
+    // `use-damage=false` does a full XGetImage per frame, which collapses on
+    // servers without working MIT-SHM (and pins the CPU everywhere else).
+    // Kept as the default; `PIXELPASS_X11_NO_DAMAGE=1` restores full-frame
+    // capture if a driver produces partial-update artifacts with damage on.
+    let use_damage = if std::env::var_os("PIXELPASS_X11_NO_DAMAGE").is_some() {
+        "use-damage=false"
+    } else {
+        "use-damage=true"
+    };
     let mut source_args = vec![
         "ximagesrc".to_string(),
-        // Full frames (no damage regions) to avoid partial-update artifacts;
-        // use-damage=true is a later CPU optimization. show-pointer matches
-        // Wayland's CursorMode::Embedded.
-        "use-damage=false".to_string(),
+        // show-pointer matches Wayland's CursorMode::Embedded.
+        use_damage.to_string(),
         "show-pointer=true".to_string(),
     ];
     if let Some(xid) = xid {