Fix TIFF export with EXIF data and I/O proxy

[jreichel-nvidia]

TIFF export with EXIF metadata (e.g. oiio::ColorSpace) creates invalid TIFF files in the presence of an I/O proxy ("TIFF directory is missing required "ImageLength" field"). The reasons is that TIFFSetDirectory() requires to read some of the previously written data, but neither writer_readproc() nor the available proxy implementations support that. Note that TIFF output does not yet use this new scheme for I/O proxy that was used for many other plugins.

I think writer_readproc() should not take any shortcuts and just forward the call to the I/O proxy.

Is there a reason why IOFile (in write mode) and IOVecOutput do not allow to re-read written data? This looks like an artificial restriction of the underlying resources. An alternative not modifiying these two implementations would be to let the TIFF output use a variant of IOVecOutput that allows re-reading and forward all data at the very end to the passed-in proxy.

Is there some policy to add assertions in all methods that are not supposed to be called? This would have helped here a lot to reduce the test case.

Tests

I did not add an explicit test. I guess the proper way would be to convert TIFF ouput to use the same I/O proxy scheme that is used by many other plugins. Small repro case is attached.
repro.cpp.txt

Checklist:

• I have read the contribution guidelines.
• [n/a] I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary). See above.
• [n/a] If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
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  nonconforming.