Resolves: tdf#101798 ResetClipRegion needs to affect cairo drawing too

regression since...

commit cb382034
Date:   Fri Jul 1 14:00:00 2016 +0200

    tdf#50613 speedup fat line drawing on linux using cairo

Change-Id: I2f1a70f102786228d52199df9960336218c6f9f9
Reviewed-on: https://gerrit.libreoffice.org/55182Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

vcl/unx/generic/gdi/salgdi.cxx

@@ -349,6 +349,9 @@ long X11SalGraphics::GetGraphicsWidth() const
 
 void X11SalGraphics::ResetClipRegion()
 {
+#if ENABLE_CAIRO_CANVAS
+    maClipRegion.SetNull();
+#endif
     mxImpl->ResetClipRegion();
 }