Compile OpenImageIO from source

[ennis]

Right now OpenImageIO is fetched via vcpkg on windows, which results in a suboptimal experience for users of the library: they need to install vcpkg, install openimageio, set the VCPKG_ROOT environment variable for build.rs, which wastes a lot of time better spent on more productive things. Also, the bindings can break on every update to the vcpkg package since we don't (can't) track a particular version of OIIO on vcpkg.

Instead, provide the option of building OIIO as part of the build process. OIIO has a lot of dependencies (OpenEXR, libtiff, plus a lot of optional ones), which will lead to some CMake insanity, but in the end the process should be transparent to the user and (hopefully) cross-platform.