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Apparently, glBindAttribLocation() does not work as planned with AMD GPUs. Therefore, all Shaders yielded black images. Replaced all attribute locations with hard-coded ones in the vertex shader and removed corresponding convenience function from cgt::Shader.

refs #382

Introducing MultiVolumeMprRenderer processor, rendering a slice through up to 3 co-registered images at the same time. Updated NeuroDemo pipeline and implemented picking of MPR plane.

* Included VoxelHierarchyMapper for empty space skipping
* Now can successfully render three volumes at the same time.