tdf#94006 - need an explicit dispose for GLContext's SystemChildWindow.

Previously we would get an explicit ~OpenGLContext - and potentially
leave FMR's around for other OGC users, now we treat the other users
properly - we need an explicit dispose() to get Window::dispose ordering
right.

Change-Id: I5edcbd73399b6db3dbcfb391570f364f9ab0c70d
Reviewed-on: https://gerrit.libreoffice.org/18412
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

include/vcl/opengl/OpenGLContext.hxx

@@ -175,6 +175,7 @@ public:
     ~OpenGLContext();
     void acquire() { mnRefCount++; }
     void release() { if ( --mnRefCount == 0 ) delete this; }
+    void dispose();
 
     void requestLegacyContext();
     void requestSingleBufferedRendering();