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| GstGLBaseFilterGstGLBaseFilter — GstBaseTransform subclass for transformin OpenGL resources | 
    GObject
    ╰── GInitiallyUnowned
        ╰── GstObject
            ╰── GstElement
                ╰── GstBaseTransform
                    ╰── GstGLBaseFilter
                        ╰── GstGLFilter
GstGLBaseFilter handles the nitty gritty details of retrieving an OpenGL
context.  It also provided some wrappers around GstBaseTransform's
start(), stop() and set_caps() virtual methods that ensure an OpenGL context
is available and current in the calling thread.
typedef struct {
  GstBaseTransformClass parent_class;
  GstGLAPI supported_gl_api;
  gboolean (*gl_start)          (GstGLBaseFilter *filter);
  void     (*gl_stop)           (GstGLBaseFilter *filter);
  gboolean (*gl_set_caps)       (GstGLBaseFilter *filter, GstCaps * incaps, GstCaps * outcaps);
} GstGLBaseFilterClass;