Improve Cue widget Keyframe rendering and usage

• For 8bit, 16bit and color channels there should be a dragable bar on the keyframe indicator. It should indicate the value for the keyframe (8bit: 0 to 255, 16bit: 0 to 16384, color: Intensity value of color). Dragging it should change the corresponding value.
• The rendering method of the timeline widget should use the drawing region hints of the drawing event. If the channel Y values are outside the range of the rendering window, the loop should continue. If the current keyframe to be rendered has an X value smaller than the X value of the rendering range, the loop should continue. If the X value of the last keyframe is smaller than the X value of the rendering range, the loop should brake.
• Transitions between key frames should be visualized. OpenGL compute shaders should be used for that. For every key frame an image should be rendered ranging from the pixel values of the last keyframe up to the current one drawing the transition given the last values, the current one and the transition method. At the end, the rednered transitions should be layed over the rendered key frames, leaving out transparent spaces for the controls of the current and last keyframe. Caching or pre-rendering should be explored as an option.

Start by locating the Cue widget's keyframe indicator and timeline widget rendering entry points. Review how drawing-event region hints and keyframe values are handled before investigating OpenGL compute-shader rendering and caching. Done means draggable channel-value bars, clipped timeline rendering, and visible transitions with transparent control spaces for the current and previous keyframes.