Blender Git Loki

Cycles: Don't tile if image area fits into tile area

Previously the check was done based on dimension of image and if any
of dimensions were larger than tile size tiling was used.

This change makes it so that if image does not exceed number of pixels
in the tile no tile will be used. Allows to render widescreen images
without tiling.

Differential Revision: https://developer.blender.org/D13206