When using preview rendering through a camera or final rendering
the `scene.render.use_motion_blur` was not respected when building
the compile directives.
This patch will when building the compile directives check if
motion blur is enabled at all. This should lead to more efficient
kernels when no motion blur is needed.
Tags: #cycles
Differential Revision: https://developer.blender.org/D4387
6e53fdc18f