GstVaapiTexture is a generic abstraction that could be moved to the
core libgstvaapi library. While doing this, no extra dependency needs
to be added. This means that a GstVaapitextureClass is now available
for any specific code that needs to be added, e.g. creation of the
underlying GL texture objects, or backend dependent ways to upload
a surface to the texture object.
Generic OpenGL data types (GLuint, GLenum) are also replaced with a
plain guint.
https://bugzilla.gnome.org/show_bug.cgi?id=736715
