Add support for OpenColorIO

[est77]

If building OpenColorIO on windows is not very difficult, adding support for it would be almost free, as I have the code written for Ramen. In appleseed.studio, users can see renders with 3d luts and gamma / exposure controls (all in the GPU, real time).
OpenColorIO could be optionally used to convert textures to / from linear.

[dictoon]

Support for OpenColorIO would be very nice to have. It's yet another dependency though and as you said we need to make sure it can be built without hassles on all platforms (I'm especially concerned about GPU support and the toolchain required to build it).

I wouldn't say support for OpenColorIO is very high priority though (compared to support for OSL, displacement or hair).

[est77]

The GPU support is all OpenGL, no need for CUDA. It can be checked on appleseed.studio at runtime if the required GL extensions are present and don't use it if not. I agree is not a high priority.

[dictoon]

The GPU support is all OpenGL, no need for CUDA. It can be checked on
appleseed.studio at runtime if the required GL extensions are present and
don't use it if not. I agree is not a high priority.

Sounds good then!