Copy simple TIFF images into buffers more directly

If we have simple, single component, tightly-packed images then
copy them directly into the allocated buffer. If we are clipping, use
memcpy to just copy the subset requested. This is primarily aimed at
accelerating the load times for TIFF volumes. Thanks to Robert
Maynard for the suggested simplications in the templated image
function.

Change-Id: I6c3b10a019fc892b1e1bbbce32841e7f946eb995
Merge topic 'tiff-reader-templating' into master

4e060c24 Copy simple TIFF images into buffers more directly