playbin3: Cleanup and refactor combiner sourcecombine
- Remove fields no longer used, or that can be replaced by smaller code
- Rename "channels" to a more meaningful "input pads"
- Directly handle/use combiner pads in the combiners instead of on the playbin3 main structure
Remove the corresponding combiner sinkpad whenever a uridecodebin3 source pad goes away
- If used, store the corresponding combiner sink pad in the SourcePad helper structure