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.
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