* add ability to specify on MeshDepthMaterial the type of depth packing one wants.

* depthRGBA -> clipZRGBA - more correct name.

* forgotten replace of depthRGBA -> clipZRGBA.

* add MeshDepthMaterial depthFormat + depthPacking.  Use equations compatible with z buffers.

* adopting MeshDepthMaterial instead of clipZRGBA.

* remove clipZRGBA shader (aka depthRGBA shader) as we can now use MeshDepthMaterial for it.

* remove some comments.

* enable skinning in MeshDepthMaterial, as it was enabled in depthRGBA material.

* remove unused shader.

* fix bug.

* fix bug in shadows with MEshDepthMaterial.

* remove unnecessary debug code.

* ensure that skinning and morphTargets propagate with MeshDepthMaterial.

* fix ssao example to use new MeshDepthMaterial.

* remove unused code.