add is_screencast to VideoSource #1239

Job	Run time
Generating protobuf	24s
Build RTC wheels (x86_64 arm64)	1m 24s
Make RTC sdist	12s
Build RTC wheels (AMD64)	1m 14s
Build RTC wheels (x86_64)	49s
Build RTC wheels (aarch64)	1m 21s
Test (ubuntu-latest, Python 3.10) / Test (ubuntu-latest, Python 3.10)	27s
Test (ubuntu-latest, Python 3.11) / Test (ubuntu-latest, Python 3.11)	27s
Test (macos-latest, Python 3.9) / Test (macos-latest, Python 3.9)	1m 8s
Test (macos-latest, Python 3.14) / Test (macos-latest, Python 3.14)	46s
Test (ubuntu-latest, Python 3.13) / Test (ubuntu-latest, Python 3.13)	28s
Test (ubuntu-latest, Python 3.12) / Test (ubuntu-latest, Python 3.12)	26s
Test (macos-latest, Python 3.12) / Test (macos-latest, Python 3.12)	38s
Test (windows-latest, Python 3.12) / Test (windows-latest, Python 3.12)	54s
Test (windows-latest, Python 3.9) / Test (windows-latest, Python 3.9)	1m 42s
Test (ubuntu-latest, Python 3.14) / Test (ubuntu-latest, Python 3.14)	26s
Test (windows-latest, Python 3.14) / Test (windows-latest, Python 3.14)	52s
Test (ubuntu-latest, Python 3.9) / Test (ubuntu-latest, Python 3.9)	39s
Publish RTC release	0s
docs	0s
	14m 17s

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