x264enc: put all theoretically possible formats in template caps

x264enc: put all theoretically possible formats in template caps

and decide at runtime which subset of these (8-bit or 10-bit video
formats) is supported. libx264 will be compiled for one of these
two options, and it is possible to switch by pointing the dynamic
linker to a different libx264 build at runtime. If we want our
template caps to be correct, they should contain all then, with
the actually supported ones determined at runtime.

https://bugzilla.gnome.org/show_bug.cgi?id=691935