fix(avhw): specify format=nv12 output for scale_vaapi filter

scale_vaapi defaults to the input sw_format (RGBZ) when no output format
is specified. h264_vaapi encoder only supports NV12/YUV formats.
Adding format=nv12 ensures the filter outputs the correct color format
for hardware encoding.

src/avhw.rs

@@ -518,7 +518,11 @@ fn build_filter_graph(
     }
 
     // scale_vaapi: hardware scaling and colourspace conversion (keeps original dimensions)
-    let mut scale_ctx = graph.add(&scale_vaapi, "scale", &format!("{width}:{height}"))?;
+    let mut scale_ctx = graph.add(
+        &scale_vaapi,
+        "scale",
+        &format!("{width}:{height}:format=nv12"),
+    )?;
     // SAFETY: scale_vaapi needs hw_device_ctx for VAAPI device access.
     unsafe {
         (*scale_ctx.as_mut_ptr()).hw_device_ctx = hw_dev.ref_clone();