Assume voxel's center at xyz = 0.5 for sampling
Assume voxel's center at xyz = 0.5 for sampling

Sampling (apart from ray entry point calculation) is now consistent
throughout the ray casting implementations. Slice viewer and the
ray casters produce consistent results.

deskvox/deskvox,deskvox/deskvox,deskvox/deskvox,deskvox/deskvox,deskvox/deskvox