vlc_opengl_interop: forward-declare vlc_gl_t

vlc_gl_t is referenced by the vlc_gl_interop structure, but only as a
pointer. Avoid including <vlc_opengl.h> and provides a forward-reference.

include/vlc_opengl_interop.h

@@ -24,6 +24,7 @@
 #include <vlc_es.h>
 #include <vlc_picture.h>
 
+typedef struct vlc_gl_t vlc_gl_t;
 struct vlc_gl_interop;
 struct vlc_video_context;