net/libvncserver: Fix build with libva

• Add new option to select whether to compile with VAAPI support, so that if multimedia/libva is already installed on the system during the build process, it is not linked to unless desired.
• Add patch to allow h.264 support to build using libva deprecated API

PR: 210533
Submitted by: cpm
Reported by: Walter Schwarzenfeld <w.schwarzenfeld@utanet.at>
Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D6974