egl: allow for EGLImage imports into VA Surfaces.

egl: allow for EGLImage imports into VA Surfaces.

Add helpers to import EGLImage objects into VA surfaces. There are
two operational modes: (i) gst_vaapi_surface_new_from_egl_image(),
which allows for implicit conversion from EGLImage to a VA surface
in native video format, and (ii) gst_vaapi_surface_new_with_egl_image(),
which exactly wraps the source EGLImage, typically in RGBA format
with linear storage.

Note: in case of (i), the EGLImage can be disposed right after the
VA surface creation call, unlike in (ii) where the user shall ensure
that the EGLImage is live until the associated VA surface is no longer
needed.

https://bugzilla.gnome.org/show_bug.cgi?id=743847