Port Image_Color and Image_Canvas to PHP8

Dear list,

I need to (try to) port Image_Color and Image_Canvas (which require 
Image_Color) to PHP 8.

For Image_Color this is not too hard. As I did for HTML_Template_IT, the 
goal is to keep compatibility from PHP 5.6 to PHP 8. The main challenge 
is to have CI and be able run the test s for a large span of PHP version 
(as PHPUnit changed a lot in between). I opened a first PR 
(https://github.com/pear/Image_Color/pull/2) to convert the test suite 
to PHPUnitPolyFill which is very handy. for such cases

I have 2 branches based on that to add GitHub actions (for CI) and to 
port the code to PHP 8 (I will open the PR after).

I haven't looked much to Image_Canvas for now but as the tests don't use 
PHPUnit (it's simple scripts), it might be a bit simpler.

As before, I would appreciate any idea on that.

Thanks in advance,
Mathieu

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Mathieu Dubois - IR - UMR 8030 équipe LABGeM
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