Blender Git Loki

UI: Reorder enums in render results image editor header

The convention is to put the label at the bottom of the enum, or in the
spot furthest away from the button. This commit reorders the items in
the "Slot", "Layer", and "Pass" menus to be consistent with this
convention. It also reorders the numbering so that higher numbers are
lower.

The original patch was by Adrian Newton (@TFS), with slight changes
and additions.

Differential Revision: https://developer.blender.org/D7142