add is_screencast to VideoSource #1249

Job	Run time
Generating protobuf	14s
Make RTC sdist	12s
Build RTC wheels (x86_64)	42s
Build RTC wheels (AMD64)	1m 13s
Build RTC wheels (x86_64 arm64)	1m 3s
Build RTC wheels (aarch64)	1m 14s
Test (ubuntu-latest, Python 3.9) / Test (ubuntu-latest, Python 3.9)	42s
Test (ubuntu-latest, Python 3.10) / Test (ubuntu-latest, Python 3.10)	26s
Test (windows-latest, Python 3.12) / Test (windows-latest, Python 3.12)	1m 7s
Test (ubuntu-latest, Python 3.13) / Test (ubuntu-latest, Python 3.13)	26s
Test (macos-latest, Python 3.9) / Test (macos-latest, Python 3.9)	1m 16s
Test (windows-latest, Python 3.14) / Test (windows-latest, Python 3.14)	1m 36s
Test (ubuntu-latest, Python 3.11) / Test (ubuntu-latest, Python 3.11)	25s
Test (macos-latest, Python 3.12) / Test (macos-latest, Python 3.12)	53s
Test (ubuntu-latest, Python 3.14) / Test (ubuntu-latest, Python 3.14)	27s
Test (macos-latest, Python 3.14) / Test (macos-latest, Python 3.14)	45s
Test (ubuntu-latest, Python 3.12) / Test (ubuntu-latest, Python 3.12)	29s
Test (windows-latest, Python 3.9) / Test (windows-latest, Python 3.9)	1m 32s
docs	0s
Publish RTC release	0s
	14m 42s

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