heap-based pixel cache allocation

I decided that it is best if the memory allocation for the image
pixels is more defensive in order to help avoid run-time memory
allocation problems.  Therefore, as of today there is this change:

	* magick/cache.c (OpenCache): While allocating the pixel cache
	from the heap, reserve enough memory to contain a full PseudoClass
	image in order to reduce the chance that there will be a memory
	allocation failure while processing the image. This also avoids
	the possibility that the image pixels will be block-copied to a
	new location due to heap memory fragmentation. If there is
	insufficient heap memory (malloc() fails), then a disk-based pixel
	cache will be used.

For most modern operating systems and modern hardware, this change
should usually not result in a change in performance since modern
operating systems use the MMU to zero-fill previously unused (clean)
memory pages when they are accessed and an allocation does not cause
an access.  Five quantums will always be allocated from virtual heap
memory rather than sometimes allocating four (DirectClass), or
sometimes allocating five (PsudeoClass), or allocating four, and then
extending the memory allocation to five (DirectClass-->PsudeoClass).

Bob
======================================
Bob Friesenhahn
[email protected]
http://www.simplesystems.org/users/bfriesen



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