Redo Calculation of Color Tinting

[oleibman]

Fix #3550. Some colors are specified in Excel by specifying a theme color to which a tint is applied. The original PHPExcel algorithm for doing this was developed by trial and error, and is good enough a lot of the time. However, for the issue at hand, the resulting color is detectably different from the calculation that Excel makes. Searching the web, I found https://gist.github.com/Mike-Honey/b36e651e9a7f1d2e1d60ce1c63b9b633 which comes much closer for the case in hand, and for all the other cases that I've looked at. That code depends on Python colorsys package; I have adapted the code from the Python gist and package into a new Php class. This doesn't agree perfectly with Excel. However, if each of the red, green, and blue components (each a value between 0 and 255 inclusive) agree within plus or minus 3 (arbitrary choice) of Excel's result, I think that is good enough.

I have added a new test member which reads from a spreadsheet with Xml altered by hand to set up several theme/tint cells. These tests use the plus-or-minus-3 criterion. They result in 100% code coverage of the new class.

Unsuprisingly, some existing tests failed with the new code. Issue2387Test reads a theme/tint font color, and is changed to use the plus-or-minus-3 criterion, comparing against the color as Excel shows it.

ColorChangeBrightness showed 9 failures with the new code. It consists of calculations not involving a spreadsheet. For that reason, I felt it was sufficient to just do an exact match test, changing the 9 old results for new results confirmed with the Python code. I also added one new test case, the one that kicked off this entire PR.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.