lavc/videotoolboxenc: Add support for HEVC with Alpha.

lavc/videotoolboxenc: Add support for HEVC with Alpha.

This change supports the "HEVC Video with Alpha" profile introduced in WWDC 2019
<https://developer.apple.com/videos/play/wwdc2019/506/>. (This change is a
partial fix for Ticket #7965.)

For example, the following command converts an animation PNG file to an HEVC
with Alpha video:
./ffmpeg -i fate-suite/apng/clock.png -c:v hevc_videotoolbox -allow_sw 1 -alpha_quality 0.75 -vtag hvc1 clock.mov

(This change uses the "HEVC Video with Alpha" profile only when the
'-alpha_quality' value is not 0 for backward compatibility.)

Signed-off-by: Hironori Bono <bouno@rouge.plala.or.jp>