use CGContextHolder in AquaSalBitmap

Change-Id: I87f5a1bc2c41c58cff747bbad82867d53ea92ce7
Reviewed-on: https://gerrit.libreoffice.org/72442
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

vcl/inc/quartz/salbmp.h

@@ -39,7 +39,7 @@ class   BitmapPalette;
 class QuartzSalBitmap : public SalBitmap
 {
 public:
-    CGContextRef                    mxGraphicContext;
+    CGContextHolder                 maGraphicContext;
     mutable CGImageRef              mxCachedImage;
     BitmapPalette                   maPalette;
     std::shared_ptr<sal_uInt8> m_pUserBuffer;