Do not allow to move required vertices in xreg option. #597

Job	Run time
mmg-debug / ci (ubuntu-20.04, on, off, on, off, int32_t)	1h 16m 56s
mmg-debug / ci (ubuntu-20.04, on, off, off, off, int32_t)	1h 34m 32s
mmg-debug / ci (ubuntu-20.04, off, off, on, off, int32_t)	1h 1m 25s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int32_t)	1h 15m 14s
mmg-debug / ci (macos-12, on, off, on, off, int32_t)	37m 5s
mmg-debug / ci (macos-12, on, off, off, off, int32_t)	1h 4m 7s
mmg-debug / ci (macos-12, off, off, on, off, int32_t)	34m 5s
mmg-debug / ci (macos-12, off, off, off, off, int32_t)	45m 9s
mmg-debug / ci (ubuntu-20.04, off, off, off, on, int32_t)	3m 31s
mmg-debug / ci (macos-12, off, off, off, on, int32_t)	6m 41s
mmg-debug / ci (ubuntu-20.04, off, on, on, off, int32_t)	1h 1m 14s
mmg-debug / ci (macos-12, off, on, on, off, int32_t)	43m 48s
mmg-debug / ci (windows-2022, off, off, off, off, int32_t)	55m 20s
mmg-debug / ci (windows-2022, on, off, off, off, int32_t)	54m 52s
mmg-debug / ci (windows-2022, off, on, off, off, int32_t)	1h 4m 21s
mmg-debug / ci (ubuntu-20.04, on, on, on, on, int64_t)	4m 20s
mmg-debug / ci (ubuntu-20.04, off, off, on, on, int64_t)	1h 9m 38s
mmg-debug / ci (ubuntu-20.04, off, off, off, off, int64_t)	2m 3s
mmg-debug / ci (macos-12, on, on, on, on, int64_t)	8m 51s
mmg-debug / ci (macos-12, off, off, off, off, int64_t)	3m 38s
mmg-debug / ci (windows-2022, on, on, off, off, int64_t)	4m 58s
mmg-debug / ci (windows-2022, off, off, off, off, int64_t)	4m 51s
mmg-debug / upload_coverage	2m 20s
	14h 38m 59s

Provide feedback