These functions provide access to a shared pool of GdkGC objects.
When a new GdkGC is needed,
gtk_gc_get() is called with the required depth,
colormap and GdkGCValues. If a GdkGC with the required properties already
exists then that is returned. If not, a new GdkGC is created.
When the GdkGC is no longer needed,
gtk_gc_release() should be called.
GdkGC * gtk_gc_get (
Gets a GdkGC with the given depth, colormap and GdkGCValues.
If a GdkGC with the given properties already exists then it is returned,
otherwise a new GdkGC is created.
The returned GdkGC should be released with
gtk_gc_release() when it is no