Move MSYS2 to GitHub Actions

[nulano]

For #3606 and #4495 (comment).

Move MSYS2 from AppVeyor to faster GHA, now that it is available.

Changes proposed in this pull request:

• Move MSYS2/MinGW from AppVeyor to faster GHA.
• Use _WIN32 instead of _MSC_VER in _imagingft.c to fix FreeType compile.
• Add libraqm-0 as alternate Raqm filename for MinGW.
• Don't add OPJ_STATIC on MinGW to fix python setup.py install: Jpeg2KDecode.c: undefined reference to `opj_stream_create' #2848. Inspired by the patch from MSYS2 repo: https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-python-pillow/002-shared-openjpeg.patch

TODO: libtiff is failing to open files for some reason (I tried both 4.1.0 and 4.0.10), disabled for now.
Looking at the MSYS2 repo, they don't seem to have a patch for libtiff, yet somehow that version passes tests. Edit: https://trac.wxwidgets.org/ticket/17286 may be related.

[nulano]

I'm not sure what is causing the difference, but I am getting epsilon around 33 for both variation tests. Checking manually, I see there is a significantly different kerning after the first letter, but the output looks correct to me.

Note: MSYS2 is currently using FreeType version 2.10.2.

[hugovk]

Please could you attach images for each case?

[nulano]

Sure.

test_variation_set_by_name (epsilon 31.5340 on line 686 of PR):
a.png (MSYS):

b.png (target - variation_adobe.png):

test_variation_set_by_axes (epsilon 39.6856 on line 720 of PR):
a.png (MSYS):

b.png (target - variation_adobe_axes.png):

Basic and Raqm layout are pixel identical. In fact, it looks like there is a bug in the tests where the used layout_engine is not being explicitly set and defaults to Raqm in both cases.

[nulano]

I decided to test Cygwin, and aside from Raqm (which was not found) and two test which should have been skipped, everything passes on this PR. It turns out that libraqm-0 is the correct name for Cygwin, but _WIN32 is not defined on Cygwin. Adding that name to the dlopen branch finds the file, but for some reason Pillow is still refusing to use it.

[hugovk]

Thank you!