Make it easier to catch code that assumes SalBitmaps are created pre-cleared

In a DBG_UTIL build fill the maContextBuffer and maUserBuffer of a
QuartzSalBitmap with an easily recognized pattern. Helps catch
situations where by coincidence they appear pre-filled with zeros most
of the time and almost everything usually looks fine.

Or is the QuartzSalBitmap implementation wrong, and SalBitmaps
*should* be created filled with zeros?

Change-Id: Ib5207779f2a47b9b7e539d8646b4128fea0ec150