x265: detect X265_BUILD

As the library symbols are suffixed with the exact build version, and
it is not clear yet whether the API will be broken or simply extended,
watch for the exact version number (which is 1)
x265: initialize picture and set picture type
contrib: vpx: update to 1.2.0
flac packetizer: use the largest possible frame size

Fixes #6476
Fixes #9442