Fix sample spacing in lens shading map saved to DNG

We were using an incorrect sample spacing of 1/N instead of 1/(N-1). This affected rendering in 3rd party tools like Adobe Photoshop, effectively shrinking the lens shading map and replicating values from the bottom-most row and right-most column.

Bug: 77657628
Test: created test pattern lens shading maps, loaded DNG in Photoshop
Change-Id: Ia814ebefeceafc72b17058a9c4cf598140d3f752