Merge IlmBase and OpenEXR Viewers binaries

Recent versions of OpenEXR provide a single source that contains IlmBase, OpenEXR itself, OpenEXR Viewers, and also PyIlmBase. PyIlmBase is not built in Debian, but the rest is by 3 different sources. Hence, let's take a step forward and unify all of them:

• built from the top-level OpenEXR project, manually enabling viewers too (apparently not enabled by upstream)
• import the binaries of src:ilmbase and src:openexr-viewer, including all their metadata (e.g. lintian overrides, man pages, doc-base, etc)

There is no change in the installed files of all the binaries. The install files are a bit more verbose now, but sadly (at least for includes) it cannot be avoided.

This series does also few extra cleanups, like removing old patches, removing autotools bits, and extra dh options. I can split them into an own MR, in case it is better for review.

A potential idea for the future could be to merge libilmbase-dev into libopenexr-dev and make the former a transitional package for the latter. It seems there are very few packages that use only the IlmBase libraries without OpenEXR, and libopenexr-dev is not that big anyway.
Another potential idea, built on top of the previous idea, is to split all the libraries in own packages at the next SONAME bump. The SONAME bump requires renaming the packages with libraries, anyway.

⚠ NOTE: in case this is accepted, after the upload it will be required to file RM bugs for src:ilmbase and src:openexr-viewer, otherwise the upload will not migrate to testing.