FFmpeg: add some syntactic sugar to get the avframe size


`frameSize` are trivial helpers to make some of the code more compact.

Pick-to: 6.12
Change-Id: I4d464cb3ab97de81d8d13847a91c5e316a05335c
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
commitae022967022fd8f3d52ca9ae65ca4c8d3a5f0132
parent908e33abc71c9274b64cb1914110ece320922765
authorTim Blechmann <tim.blechmann@qt.io>
committerTim Blechmann <tim.blechmann@qt.io>
commit timeMon, 19 Oct 2026 04:05:50 +0000