feat: add KWin/KDE Plasma screen capture via xdg-desktop-portal ScreenCast + PipeWire

Add a second capture backend for compositors without wlr-screencopy
(KWin, GNOME, etc.) using the xdg-desktop-portal ScreenCast interface
and PipeWire DMA-BUF streaming.

New files:
- src/backend_detect.rs: auto-detect wlr-screencopy vs portal backend
- src/cap_portal.rs: Portal session setup + PipeWire DMA-BUF thread
- src/state_portal.rs: StatePortal encoder pipeline (DMA-BUF → VAAPI)

Changes:
- Cargo.toml: add ashpd 0.13, tokio 1, pipewire 0.9, libspa 0.9,
  crossbeam-channel 0.5
- src/args.rs: add --backend CLI flag
- src/avhw.rs: extract create_encoder() from inline State code
- src/main.rs: route to portal or wlr-screencopy based on backend
- src/state.rs: fix params.destroy() on dup failure, cleanup
  in_flight_surface on copy fail, use create_encoder()
- tests/integration_test.rs: add --backend flag tests

src/avhw.rs

@@ -7,7 +7,7 @@ use ffmpeg_next as ff;
 use ffmpeg_next::ffi;
 use ffmpeg_next::packet::Mut as _;
 
-use crate::transform::Transform;
+use crate::transform::{transpose_if_transform_transposed, Transform};
 
 // ---------------------------------------------------------------------------
 // AvHwDevCtx
@@ -463,6 +463,46 @@ impl EncState {
     }
 }
 
+// ---------------------------------------------------------------------------
+// Shared encoder creation (used by both wlr-screencopy and portal paths)
+// ---------------------------------------------------------------------------
+
+/// Create a fully configured encoder with VAAPI hardware acceleration.
+///
+/// Convenience wrapper around [`EncState::new`] that computes default values
+/// for `bitrate` and `gop_size` when not provided, and handles encoder dimension
+/// transposition for rotated/transformed outputs.
+#[allow(clippy::too_many_arguments)]
+pub fn create_encoder(
+    drm_device: &Path,
+    output_path: &Path,
+    width: u32,
+    height: u32,
+    fps: u32,
+    transform: Transform,
+    bitrate: Option<u64>,
+    gop_size: Option<u32>,
+) -> Result<EncState> {
+    let (enc_w, enc_h) =
+        transpose_if_transform_transposed(transform, width as i32, height as i32);
+    let actual_bitrate = bitrate.unwrap_or_else(|| {
+        2 * (width as u64) * (height as u64) * (fps as u64) / 100
+    });
+    let actual_gop_size = gop_size.unwrap_or(fps);
+    EncState::new(
+        drm_device,
+        output_path,
+        width,
+        height,
+        enc_w as u32,
+        enc_h as u32,
+        actual_bitrate,
+        actual_gop_size,
+        fps,
+        transform,
+    )
+}
+
 // ---------------------------------------------------------------------------
 // Filter graph (inline)
 // ---------------------------------------------------------------------------
@@ -535,31 +575,30 @@ fn build_filter_graph(
     src_ctx.link(0, &mut scale_ctx, 0);
 
     match transform {
-        Transform::Normal90 | Transform::Normal270 => {
+        Transform::Normal => {
+            scale_ctx.link(0, &mut sink_ctx, 0);
+        }
+        other => {
             let transpose = ff::filter::find("transpose_vaapi")
                 .ok_or_else(|| anyhow::anyhow!("filter 'transpose_vaapi' not found"))?;
-            let dir_val = match transform {
+            let dir_val = match other {
                 Transform::Normal90 => "1",
+                Transform::Normal180 => "4",
                 Transform::Normal270 => "2",
-                _ => unreachable!(),
+                Transform::Flipped => "5",
+                Transform::Flipped90 => "3",
+                Transform::Flipped180 => "6",
+                Transform::Flipped270 => "0",
+                Transform::Normal => unreachable!(),
             };
-            let mut trans_ctx = graph.add(&transpose, "transpose", &format!("dir={dir_val}"))?;
-            // SAFETY: transpose_vaapi needs hw_device_ctx for VAAPI device access.
+            let mut trans_ctx =
+                graph.add(&transpose, "transpose", &format!("dir={dir_val}"))?;
             unsafe {
                 (*trans_ctx.as_mut_ptr()).hw_device_ctx = hw_dev.ref_clone();
             }
             scale_ctx.link(0, &mut trans_ctx, 0);
             trans_ctx.link(0, &mut sink_ctx, 0);
         }
-        Transform::Normal180 => {
-            tracing::warn!(
-                "Normal180 transform detected; rotation correction deferred to follow-up"
-            );
-            scale_ctx.link(0, &mut sink_ctx, 0);
-        }
-        _ => {
-            scale_ctx.link(0, &mut sink_ctx, 0);
-        }
     }
 
     graph