add is_screencast to VideoSource #1238

Job	Run time
Build RTC wheels (x86_64 arm64)	1m 9s
Build RTC wheels (aarch64)	1m 17s
Build RTC wheels (x86_64)	48s
Build RTC wheels (AMD64)	3m 12s
Make RTC sdist	10s
Generating protobuf	16s
Test (ubuntu-latest, Python 3.10) / Test (ubuntu-latest, Python 3.10)	26s
Test (macos-latest, Python 3.14) / Test (macos-latest, Python 3.14)	48s
Test (windows-latest, Python 3.14) / Test (windows-latest, Python 3.14)	56s
Test (ubuntu-latest, Python 3.11) / Test (ubuntu-latest, Python 3.11)	27s
Test (windows-latest, Python 3.9) / Test (windows-latest, Python 3.9)	1m 57s
Test (ubuntu-latest, Python 3.13) / Test (ubuntu-latest, Python 3.13)	29s
Test (ubuntu-latest, Python 3.9) / Test (ubuntu-latest, Python 3.9)	32s
Test (macos-latest, Python 3.12) / Test (macos-latest, Python 3.12)	44s
Test (ubuntu-latest, Python 3.14) / Test (ubuntu-latest, Python 3.14)	27s
Test (windows-latest, Python 3.12) / Test (windows-latest, Python 3.12)	54s
Test (ubuntu-latest, Python 3.12) / Test (ubuntu-latest, Python 3.12)	26s
Test (macos-latest, Python 3.9) / Test (macos-latest, Python 3.9)	1m 12s
Publish RTC release	0s
docs	0s
	16m 10s

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