Support loading the alpha channel of videos. (#13564) #734

Job	Run time
test-stable (linux, 3.12, 12.1, stable)	1d 0h 0m 0s
test-stable (linux, 3.11, 12.1, stable)	1d 0h 0m 0s
test-stable (linux, 3.10, 12.1, stable)	1d 0h 0m 0s
test-unix-nightly (linux, 3.11, 12.1, nightly)	1d 0h 0m 0s
	4d 0h 0m 0s

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