Add Depth stage to polydata fragment shaders.

The depth peelers currently assume gl_FragCoord.z is always the depth,
but the imposter shaders in Domains/Chemistry modify the depth.

This patch adds a //VTK::Depth::Impl hook into the fragment shader
template, before any depth peeling processing is applied. The depth
is stored into gl_FragDepth, and all depth processing now uses this
variable instead of gl_FragCoord.z.