Matroska: scan for duration when not provided by metadata #295

Job	Run time
Static Analysis	1m 4s
Lint (5.6, false)	30s
Lint (5.3, false)	36s
Lint (7.1, false)	12s
Lint (7.0, false)	26s
Lint (5.5, false)	31s
Lint (5.4, false)	31s
Lint (8.1, false)	11s
Lint (8.5, true, --ignore-platform-reqs)	13s
Lint (7.3, false)	15s
Lint (7.2, false)	12s
Lint (8.2, false)	16s
Lint (8.0, false)	12s
Lint (8.4, false)	11s
Lint (8.3, false)	12s
Lint (7.4, false)	13s
	5m 45s

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