Add new generic helper functions gst_vaapi_texture_new_wrapped()
and gst_vaapi_texture_new() to create a texture without having
the caller to uselessly check for the display type himself. i.e.
internally, there is now a GstVaapiDisplayClass hook to create
textures, and the actual backend implementation fills it in.
This is a simplification in view to supporting EGL.
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
