Encoder: emit S-frames at key-frame positions

[mabenjak]

Describe the requested feature / task

The encoder can curently only generate switch frames at the last position (in display order) of the coding structure for random access:

        Inter         Inter          Inter         Inter
               Inter                        Inter
  CLK                        SWITCH                        SWITCH 

The following structure seems more useful for adaptive streaming use-cases:

        Inter         Inter                 Inter         Inter
               Inter                               Inter
  CLK                        Inter   SWITCH                        Inter 

Describe how the feature will be implemented / how the task will be completed

Introduce a new encoder parameter --sframe-replace-kf=N that replaces key frames with S-frames in a repeating pattern:

• 0: disabled (default) — key frames remain key frames
• 1: K-S-K-S… — replace every other key frame with an S-frame
• 2: K-S-S-K-S-S… — replace two consecutive key frames with S-frames between actual key frames
• N: replace N consecutive key frames with S-frames between actual key frames