Fixes for tickets #961, #1455 and #1447: Decoding and verifying of GDR conformance streams

This branch includes fixes to closely related problems of GDR decoding and conformance testing:

1. Corrections of generation of missing reference pictures to allow decoding of sequences starting with GDR and recovery POC > 0 (ticket #1455)
2. Fix to the -SkipFrames (-s) parameter to allow testing of GDR (or CRA) later in a sequence. (ticket #961)
3. Suppress YUV and OPL output before reaching a recovery point, as discussed during last JVET meeting (ticket #1447)