Volatile fields were introduced to the RenderResult struct years ago[1].
However, volatile is most likely not doing what it was intended to do
in this instance, and is problematic when moving files to c++ (see
discussion from D13962). There are complex rules around what happens to
these fields but none of them guarantee what the above commit alluded to.
This patch drops the volatile and cleans up the APIs surrounding it.
[1] rB7930c40051ef1b1a26140629cf1299aa89eed859
Passing on all platforms:
https://builder.blender.org/admin/#/builders/18/builds/338
Differential Revision: https://developer.blender.org/D14298
