LibVideo/VP9: Consolidate frame size calculations

This moves all the frame size calculation to `FrameContext`, where the
subsampling is easily accessible to determine the size for each plane.
The internal framebuffer size has also been reduced to the exact frame
size that is output.

Userland/Libraries/LibVideo/VP9/Utilities.h

@@ -119,4 +119,9 @@ inline u8 transform_size_to_sub_blocks(TransformSize transform_size)
     return 1 << transform_size;
 }
 
+inline u32 y_size_to_uv_size(bool subsampled, u32 size)
+{
+    return (size + subsampled) >> subsampled;
+}
+
 }