file-exr: Add initial implementation (loader)
This is a basic implementation of an OpenEXR loader. This "infrastructure" is required for any further work. It consists of: * The build system changes. * A C wrapper around the OpenEXR library, which is necessary as it's not possible to intermix GIMP's code with C++ code. * A basic image loader. Chroma is not supported currently, and some other weird files like multi-view files are unsupported. These can be added when necessary. There is no UI, but it should be straightforward to add new features like this on top of this work.
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