gstglwindow_win32: share opengl context in correct order
gstglwindow_win32: share opengl context in correct order

Allow to use the opengl context sharing feature if the original
context has been used already.
This is only possible on win32 with wglShareLists because on other
backends, the sharing is made when a context is created.
This new feature is only usefull when using external context
because gst-gl internally makes sure that the orginal context is not
already used when sharing it.

Fix bug #611067

ystreet/gst-plugins-gl,ystreet/gst-plugins-gl,ystreet/gst-plugins-gl,mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl,mapmapteam/gst-plugins-gl