Improve scene detection (#2710)

Current fast scene detection in rav1e relatively slow to compared methods of scene detection, and have proclivity to show false results or don't detect scene changes where they appear

This pr reworks fast scene detection algorithm, making it faster, better, and more accurate

Achieved goals are:

    Faster decision making ( Both less and more efficient computations )
    More accurate scene detection, by adjusting threshold based on previous frames
    Frame downscale for faster decisions.

Author: master-of-zen

src/api/channel/by_gop.rs

@@ -39,15 +39,15 @@ impl<T: Pixel> SubGop<T> {
 */
 
 // TODO: Make the detector logic fitting the model
-struct SceneChange {
+struct SceneChange<T: Pixel> {
   frames: usize,
   pyramid_size: usize,
   processed: u64,
   last_keyframe: u64,
-  detector: SceneChangeDetector,
+  detector: SceneChangeDetector<T>,
 }
 
-impl SceneChange {
+impl<T: Pixel> SceneChange<T> {
   fn new(pyramid_size: usize, enc: &EncoderConfig) -> Self {
     let seq = Arc::new(Sequence::new(enc));
 
@@ -56,17 +56,14 @@ impl SceneChange {
       CpuFeatureLevel::default(),
       pyramid_size,
       seq,
-      true,
     );
 
     Self { frames: 0, pyramid_size, processed: 0, last_keyframe: 0, detector }
   }
 
   // Tell where to split the lookahead
   //
-  fn split<T: Pixel>(
-    &mut self, lookahead: &[Arc<Frame<T>>],
-  ) -> Option<(usize, bool)> {
+  fn split(&mut self, lookahead: &[Arc<Frame<T>>]) -> Option<(usize, bool)> {
     self.processed += 1;
 
     let new_gop = self.detector.analyze_next_frame(

src/api/internal.rs

@@ -246,7 +246,7 @@ pub(crate) struct ContextInner<T: Pixel> {
   gop_output_frameno_start: BTreeMap<u64, u64>,
   /// Maps `output_frameno` to `gop_input_frameno_start`.
   pub(crate) gop_input_frameno_start: BTreeMap<u64, u64>,
-  keyframe_detector: SceneChangeDetector,
+  keyframe_detector: SceneChangeDetector<T>,
   pub(crate) config: Arc<EncoderConfig>,
   seq: Arc<Sequence>,
   pub(crate) rc_state: RCState,
@@ -291,7 +291,6 @@ impl<T: Pixel> ContextInner<T> {
         CpuFeatureLevel::default(),
         lookahead_distance,
         seq.clone(),
-        true,
       ),
       config: Arc::new(*enc),
       seq,