perf(video): move tier scaling to ffmpeg

The frame pump now submits crop-sized BGRA frames; FFmpeg scales them to final tier dimensions. Bundled scaler benchmark: 300 608x1080 to 1080x1920 frames in 0.435s (scaler-only).

Reference: https://ffmpeg.org/ffmpeg-filters.html#scale-1

ViewModels/MainViewModel.cs

@@ -654,19 +654,21 @@ public partial class MainViewModel : ViewModelBase
 
     // The tier's crop rect over the 1920x1080 master, integer-aligned (the VM's
     // OutputRect* are doubles — the vertical 607.5 half-pixel crop rounds to 608).
+    // FFmpeg, not the compositor, scales this crop to the tier's final dimensions.
     // internal = test seam (the zoom layer must NOT change the output crop — the
     // box is a composited layer, never the crop source).
     internal CompositorOptions BuildCompositorOptions()
     {
-        var quality = SelectedQuality;
+        var sourceRectWidth = (int)Math.Round(OutputRectWidth);
+        var sourceRectHeight = (int)Math.Round(OutputRectHeight);
         return new CompositorOptions
         {
             SourceRectX = (int)Math.Round(OutputRectX),
             SourceRectY = (int)Math.Round(OutputRectY),
-            SourceRectWidth = (int)Math.Round(OutputRectWidth),
-            SourceRectHeight = (int)Math.Round(OutputRectHeight),
-            OutputWidth = quality.Width,
-            OutputHeight = quality.Height,
+            SourceRectWidth = sourceRectWidth,
+            SourceRectHeight = sourceRectHeight,
+            OutputWidth = sourceRectWidth,
+            OutputHeight = sourceRectHeight,
         };
     }    // The credit's cadence, timing, placement and licence gate all live in
     // BrandFlashPresenter; MainViewModel only routes it to the frame pump and the