Vendor specific feature support

[XinfengZhang]

I open this issue to collect your input from different perspective. Current libva will ignore the vendor differences and depend on the capability query function for different feature and settings. but there are some restrictions, such as the discussion in https://github.com/intel/libva/pull/573/files#r802206720 different vendor may have different HW to implement it.

another one example is : https://github.com/intel/libva/pull/529 the look ahead interfaces. it also have vendor specific algorithm support.

of course, seems current there are no much gaps, because most of the case we could use the common definitions + documentation to describe these feature with common sense.

another purpose of this issue is to collect whether there are other specific feature need different definitions for different vendors.