THIS CHANGE BREAKS MATERIALS.
This adds getUserTime() in shaders/materials, which returns the time
in second since Renderer::resetUserTime() was called.
Two values are provided, the time in second encoded as a float and
the difference between that and the double value, which together allows
to perform high precision time computation when needed.
This change allows longer running animations in materials. Using only
the float value, give millisecond resolution for more than 4h.
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