General: FFmpeg conversion also check attribute string length

[iLLiCiTiT]

Brief description

Conversion for ffmpeg is also checking length of string stored in exr attributes because ffmpeg has limit up to 8196 characters.

Changes

• information about input file from oiiotool are loaded using xml format
  • this gives more and more precise information about input
• all attributes with string values longer than 8196 are removed from exr to be able pass it to ffmpeg
• few smaller tweaks of oiiotool conversion
  • oiiotool loads only channels that are used for output so works a little bit faster with reducing memory leak issues
  • function for converting does not require frame start/env arguments

Testing notes:

1. Try publish exrs with string attributes which have value longer than 8196 characters (e.g. cryptomatte stores whole manifest)
2. It should be able to create review and thumbnail just fine

[mkolar]

Task linked: OP-2573 Multilayer exrs conversion does not work all the time

[kalisp]

Fixes issue with testing .exr