JpegImageConverter plugin, JPEG support in StbImageConverter

[mosra]

A new plugin creating JPEGs out of your 8-bit RGB and grayscale image data. The test is failing on small input (while I'm pretty sure it works on larger input), needs further investigation.

Opening a PR in case someone finds this code handy (and also so it doesn't get lost in my abandoned branches).

TODO:

• Update copyright year!
• Make it build on the CI with just DebugTools enabled and not all its optional dependencies
• Implement JPEG detection in AbstractImageConverter (note to self: there's a jpeg branch of magnum for this) mosra/magnum@c60217a

[mosra]

Revived this PR now, hopefully on track for 2018.08.

Turns out the original code was not taking pixel storage properties correctly into account, which explains the huge perceived difference in output. Now testing both with "ground truth" data and using DebugTools::CompareImage to ensure reasonable error margin when the output is with 100% quality setting. After stb_image_write update, part of this PR is also JPEG support in StbImageConverter.

The main blocker now is an ability to find DebugTools without requiring all optional dependencies it has so the CI doesn't need to spend time building things that are not needed.

[mosra]

In order to unblock the progress, I merged the majority of the additions into master (e2c4ae2...b40ef6f). The only remaining bit is the additional check using DebugTools::CompareImage, blocked by the above.

[codecov-io]

Codecov Report

Merging #26 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #26   +/-   ##
=======================================
  Coverage   89.12%   89.12%           
=======================================
  Files          43       43           
  Lines        4265     4265           
=======================================
  Hits         3801     3801           
  Misses        464      464

Impacted Files	Coverage Δ
src/MagnumPlugins/ColladaImporter/Utility.h	100% <ø> (ø)	⬆️
src/MagnumPlugins/ColladaImporter/ColladaType.h	100% <ø> (ø)	⬆️
.../MagnumPlugins/ColladaImporter/ColladaImporter.cpp	92.81% <100%> (ø)	⬆️

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