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.

scripts/travis.sh

@@ -1,37 +0,0 @@
-#!/bin/bash
-
-set -e
-
-if [[ $ENABLE_SANITIZERS ]]; then
-    LOCAL_CMAKE_EXTRA_ARGS="-DCMAKE_C_FLAGS=-fsanitize=$ENABLE_SANITIZERS"
-    LOCAL_MAKE_EXTRA_ARGS="CFLAGS=-fsanitize=$ENABLE_SANITIZERS LDFLAGS=-fsanitize=$ENABLE_SANITIZERS"
-fi
-
-if [[ $USE_CMAKE ]]; then
-    echo "$0: using cmake + make:" $LOCAL_CMAKE_EXTRA_ARGS $EXTRA_ARGS
-    mkdir build-cmake
-    cd build-cmake
-    cmake $LOCAL_CMAKE_EXTRA_ARGS $EXTRA_ARGS ..
-    make
-    [[ $DISABLE_TESTS ]] || make test
-    make clean
-fi
-
-if [[ $USE_CONFIGURE ]]; then
-    mkdir build-configure
-    cd build-configure
-    echo "$0: using configure + make:" $LOCAL_MAKE_EXTRA_ARGS $EXTRA_ARGS
-    ../configure $LOCAL_MAKE_EXTRA_ARGS $EXTRA_ARGS
-    make
-    [[ $DISABLE_TESTS ]] || make test
-    make clean
-    make distclean
-fi
-
-if [[ $USE_LEGACY_MAKEFILES ]]; then
-    echo "$0: using scripts/makefile.$CC:" $LOCAL_MAKE_EXTRA_ARGS $EXTRA_ARGS
-    make -f scripts/makefile.$CC $LOCAL_MAKE_EXTRA_ARGS $EXTRA_ARGS
-    [[ $DISABLE_TESTS ]] || make -f scripts/makefile.$CC $LOCAL_MAKE_EXTRA_ARGS $EXTRA_ARGS test
-    make -f scripts/makefile.$CC $LOCAL_MAKE_EXTRA_ARGS $EXTRA_ARGS clean
-    # TODO: use scripts/makefile.std, etc.
-fi