Make FlyPaletteComposite contents use native double buffering

The FlyoutPaletteComposite is instantiated with style flag
SWT.DOUBLE_BUFFERED to use native double buffering. It's contained
TitleCanvas and ButtonCanvas do not use that flag. In consequence, their
LightweightSystem creates a BufferedGraphicsSource instead of a
NativeGraphicsSource for them.

This change adapts the TitleCanvas and ButtonCanvas instantiation to add
the SWT.DOUBLE_BUFFERED style, such that they use native double
buffering.

Author: HeikoKlare

org.eclipse.gef/src/org/eclipse/gef/ui/palette/FlyoutPaletteComposite.java

@@ -1168,7 +1168,7 @@ private class ButtonCanvas extends Canvas {
 		private LightweightSystem lws;
 
 		public ButtonCanvas(Composite parent) {
-			super(parent, SWT.NO_REDRAW_RESIZE | SWT.NO_BACKGROUND);
+			super(parent, SWT.NO_REDRAW_RESIZE | SWT.NO_BACKGROUND | SWT.DOUBLE_BUFFERED);
 			init();
 			provideAccSupport();
 		}
@@ -1314,7 +1314,7 @@ private class TitleCanvas extends Canvas {
 		private LightweightSystem lws;
 
 		public TitleCanvas(Composite parent, boolean horizontal) {
-			super(parent, SWT.NO_REDRAW_RESIZE | SWT.NO_BACKGROUND);
+			super(parent, SWT.NO_REDRAW_RESIZE | SWT.NO_BACKGROUND | SWT.DOUBLE_BUFFERED);
 			init(horizontal);
 			provideAccSupport();
 		}