Enhance VideoEditor with previous and next frame tracking point visualization

This commit adds functionality to the VideoEditor class to render tracking points from the previous and next frames with 50% alpha blending. Red circles indicate previous frame points, while green circles represent next frame points, improving the visual feedback during video editing. Additionally, the feedback message duration has been reduced for a more responsive user experience.

croppa/spec.md

@@ -55,6 +55,7 @@ That coordinate is to be mapped to the bottom left corner of the original raw un
 - **Blue cross**: Shows computed tracking position
 - **Automatic interpolation**: Tracks between keyframes
 - **Crop follows**: Crop area centers on tracked object
+- **Display** Points are rendered as blue dots per frame, in addition dots are rendered on each frame for each dot on the previous (in red) and next (in green) frame
 
 ### Markers and Looping
 - **1**: Set cut start marker at current frame