vaapi: dynamic load va-x11 and va-gl

TODO: system may have no x11, so x11 should dynamically load too
use qopengl.h instead of gl.h? check glx or gles and use va-egl
vaapi: dynamic load x11. use qt opengl header
Merge pull request #240 from wang-bin/va

va: dynamic load va and x11 libs to let vaapi work on variant evironments