[gtk] Destroy cached image at sizee handle

This commit ensures the cleanup of the image handle cached by size on
destruction of the Image.

Author: akoch-yatta

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/Image.java

@@ -928,6 +928,7 @@ void destroy() {
 	if (surface != 0) Cairo.cairo_surface_destroy(surface);
 	surface = mask = 0;
 	memGC = null;
+	cachedImageAtSize.destroy();
 }
 
 private CachedImageAtSize cachedImageAtSize = new CachedImageAtSize();