m4macros/pythondev.m4: Python 3 compatibility
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.
file-exr: Fix a possible crash upon failure
build: Lower required version of OpenEXR library
app: don't crash when adjusting quality during an export to jpeg

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Merge branch 'master' of git://git.gnome.org/gimp