Simplify the depth transformation in the scene graph.

Simplify the depth transformation in the scene graph.

Use default depth function GL_LESS and depth clear value 1, and
set the near and far clipping planes to -1 and 1. This change
will make the transformations a bit easier for people who
implement custom 3D geometry since [-1, 1] is the typical z-range
after a projection transformation. The change has no visual impact
on 2D geometry.

Change-Id: I75d4a8acc15131ffaa5d13a749e42dffbc1d09db
Reviewed-by: Glenn Watson <glenn.watson@nokia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>