Blender Git Loki

Cycles: Fix missing OpenCL extensions in certain cases

If extensions string is longer than 1024 then the old code would have
reported empty string instead of extensions.

Now the code does dynamic string allocation to store result of request,
similar to what is done in `OpenCLInfo::get_hardware_id`.

The code looks a bit ugly, but it didn't really change much with this
patch. In other words, the code can become more modern and clear, but
it is considered to be outside of the scope of this change.

Differential Revision: https://developer.blender.org/D10135