Update configuration for Travis CI; add configuration for AppVeyor CI
Exclude the branch "libpng16" from testing. It's identical to "master".
Also exclude the other (non-current) "libpng[0-1][0-7]" branches.
Parallelize the builds and the unit tests.
For Travis CI, update the build matrix as follows:
* On Linux, use both clang and gcc.
* On macOS, use clang only, with and without Xcode.
For AppVeyor CI, add a build matrix as follows:
* On Windows, use Microsoft Visual Studio 2019 on x86, x64 and arm64.
* Also on Windows, use MSYS2 on x86 and x64.
Use the new scripts/ci_*.sh; remove the old scripts/travis.sh.
3 files changed
