Blender Git Loki

Shading: Add a clamp option to the Map Range node.

If the option is enabled, the output is clamped to the target range.
The target range is [To Min, To Max]. The option is enabled by default.

The clamp option is implemented in EEVEE by linking to the `clamp_value`
GLSL function. And it is implemented in Cycles using a graph expand
function.

Reviewers: brecht, JacquesLucke

Differential Revision: https://developer.blender.org/D5477