add is_screencast to VideoSource #1237

Job	Run time
Make RTC sdist	12s
Generating protobuf	22s
Build RTC wheels (x86_64)	40s
Build RTC wheels (AMD64)	1m 36s
Build RTC wheels (x86_64 arm64)	51s
Build RTC wheels (aarch64)	1m 38s
Test (windows-latest, Python 3.12) / Test (windows-latest, Python 3.12)	54s
Test (ubuntu-latest, Python 3.9) / Test (ubuntu-latest, Python 3.9)	33s
Test (windows-latest, Python 3.9) / Test (windows-latest, Python 3.9)	1m 31s
Test (ubuntu-latest, Python 3.13) / Test (ubuntu-latest, Python 3.13)	33s
Test (macos-latest, Python 3.14) / Test (macos-latest, Python 3.14)	51s
Test (ubuntu-latest, Python 3.10) / Test (ubuntu-latest, Python 3.10)	29s
Test (ubuntu-latest, Python 3.14) / Test (ubuntu-latest, Python 3.14)	28s
Test (windows-latest, Python 3.14) / Test (windows-latest, Python 3.14)	1m 20s
Test (macos-latest, Python 3.12) / Test (macos-latest, Python 3.12)	47s
Test (ubuntu-latest, Python 3.12) / Test (ubuntu-latest, Python 3.12)	28s
Test (ubuntu-latest, Python 3.11) / Test (ubuntu-latest, Python 3.11)	26s
Test (macos-latest, Python 3.9) / Test (macos-latest, Python 3.9)	1m 12s
Publish RTC release	-1s
docs	0s
	14m 50s

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