This commit also upgrades related dependencies:

- OpenEXR to 2.5.3; previous versions have a broken pkg-config file,
which makes it unlocatable by GMic-Qt.
- ilmbase is now dropped, as it is bundled by OpenEXR.
- fftw3 has now threads enabled on Linux; it is a required library by
GMic.

I also cleaned up OpenEXR patches, as they have been upstreamed. GMic
patches related to threading were removed (see above), and the desktop
entry patch was updated to match the new source.

BUG:412617

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