Bug 875441 - Don't keep reapplying the 2x scale in HiDPI mode when drawing the titlebar buttons. r=mattwoodrow

widget/cocoa/nsChildView.mm

@@ -2057,6 +2057,7 @@ nsChildView::UpdateTitlebarImageBuffer()
   ClearRegion(mTitlebarImageBuffer, dirtyTitlebarRegion);
 
   CGContextRef ctx = mTitlebarImageBuffer->GetCGContext();
+  CGContextSaveGState(ctx);
 
   double scale = BackingScaleFactor();
   CGContextScaleCTM(ctx, scale, scale);
@@ -2113,6 +2114,7 @@ nsChildView::UpdateTitlebarImageBuffer()
                         DevPixelsToCocoaPoints(1));
 
   [NSGraphicsContext setCurrentContext:oldContext];
+  CGContextRestoreGState(ctx);
 
   mUpdatedTitlebarRegion.Or(mUpdatedTitlebarRegion, dirtyTitlebarRegion);
 }