This was disabled to avoid updating the geometry every time when the
material includes displacement, because there was no way to distinguish
between surface shader and displacement updates.
As a solution, we now compute an MD5 hash of the nodes linked to the
displacement socket, and only update the mesh if that changes.
Differential Revision: https://developer.blender.org/D3018
Fix#29475: remove node from properties editor crash on windows. This was a bug
in the UI code, which code access removed data.
Fix OpenCL still being used in a case where Experimental was disabled.
Fix msvc debug warning in md5 code.
* Compute MD5 hash to deal with nvidia opencl compiler cache not recognizing
changes in #included files, makes it possible to do kernel compile only
once and remember it for the next time blender is started.
* Kernel tweak to compile with ATI/linux. Enabling any more functionality than
simple clay render still chokes the compiler though, without a specific error
message ..