Blender Git Loki

Fix T69044: OpenCL fail due to bad fract function.

The fract function in OpenCL does more than just return the fraction.
It also writes the floor to the second argument. Which wasn't put
in consideration.

Instead, we use a simple `a - floor(a)` like the Math node.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5553