VLCOpenGLES2VideoView: remove EAGL context in close

Fix a crash where the destructor of the VLCOpenGLES2VideoView
(.cxx_destruct) is calling the destructor of the EAGL context, which
leads into unknown stacktraces in the EAGLContext_setDebugLabel symbol.

The setup of the crash needs using CVPixelBuffer backed by an IOSurface,
so typically a filter using the VLCCVOpenGLProvider offscreen OpenGL
implementation, which seems to imply some live-binding is still in use
if we don't close the context soon enough.

modules/video_output/apple/VLCOpenGLES2VideoView.m

@@ -290,6 +290,11 @@ static void Close(vlc_gl_t *gl)
     _eaglEnabled = NO;
     vlc_mutex_unlock(&_mutex);
 
+    /* We can drop the context as fast as possible since we don't use it
+     * anymore. It avoids crashes with the context modifying destroyed
+     * resources when it is being deallocated in the main thread. */
+    _eaglContext = nil;
+
     /* This cannot be a synchronous dispatch because player is usually running
      * in the main thread and block the main thread unless we accept our fate
      * and exit here. */