vo_opengl: fix redrawing with hardware decoding

unmap_current_image() is called after rendering. This essentially
invalidates the textures, so we can't assume that the image is still
present.

Also see PR #3773.

video/out/opengl/video.c

@@ -961,6 +961,7 @@ static void unmap_current_image(struct gl_video *p)
             p->hwdec->driver->unmap(p->hwdec);
         memset(vimg->planes, 0, sizeof(vimg->planes));
         vimg->hwdec_mapped = false;
+        vimg->id = 0; // needs to be mapped again
     }
 }