Document the processing pipeline applied to images at load time

Document the processing pipeline applied to images at load time

[problem] Dali applications cannot use image attributes with confidence
and they are not obeyed in our image loading code.
[cause] There is no clear wording on how the use of ImageAttributes is
expected to affect the loading of images by Dali in our documentation.
[solution] Clearly lay out an order of operations and the handling of
special parameter values (zero dimensions).

Change-Id: I30399cc17dc6581606fa85886ab8b51c79e4a94f
Signed-off-by: Andrew Cox <andrew.cox@partner.samsung.com>