Improve depth interpolation in reference renderer

Improve depth interpolation in reference renderer

This change reformulates depth value interpolation to exactly preserve
constant depth across a triangle. Old interpolation code exhibited
serious Z-fighting with two (Z-)overlapping screen-aligned triangles.

In addition barycentric precision is slightly improved by actually
dividing with the sum instead of sharing pre-computed 1/sum.

A simple test for constant depth and varying interpolation is provided.

Change-Id: If6229f9652bcf3dbe1d273ed8e131b175c06dfc2