Blender Git Loki

Cycles: Support "precompiled" headers in include expansion algorithm

The idea here is that it is possible to mark certain include statements
as "precompiled" which means all subsequent includes of that file will
be replaced with an empty string.

This is a way to deal with tricky include pattern happening in single
program OpenCL split kernel which was including bunch of headers about
10 times.

This brings preprocessing time from ~1sec to ~0.1sec on my laptop.