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