Add support for zstd compression

[probonopd]

We are trying to standardize on zstd (no pun intended). So if libappiage doesn't support it yet, it would be highly appreciated if someone could add it.

@philmb3487 over at AppImage/appimagetool#68 (comment):

Some people are generating AppImages with zstd compression, and that is not supported out of the box by libappimage.
Now, that is fine for executing AppImages, but file thumbnailers such as KDE's Dolphin use libappimage.so directly to peek at the .DirIcon information, which fails for zstd images.

Would it be possible to add this support?

Arch Linux seems to enable fuse to get that support, squashfuse then needs to be compiled with zstd support turned on. That's a lot of switches that might or might not be enabled, depending on the distribution, and I think thumbnails should always be available in any case.

See my findings here,
ivan-hc/Steam-appimage#27 (comment)

So, I propose either,

1. better zstd support out of the box,
2. document what needs to be done if Thumbnails cannot be shown on some appimages (out of the box it would be fine with xz, zip images,

Squashfs image uses (null) compression, this version supports only xz, zlib.
ERROR: appimage_read_file_into_buffer_following_symlinks : sqfs_open_image error: /home/phil/Applications/Steam-1.0.0.81-2-3-x86_64.appimage

[probonopd]

There is a pull request that did not get merged so far, maybe it can be reactivated:

• add zstd compression #158

[probonopd]

Currently we are using vasi/squashfuse@1f98030 from 2016. We should probably use a more recent version.

The changes would need to be made around here:

libappimage/cmake/dependencies.cmake

Lines 85 to 104 in e56c21b

	ExternalProject_Add(
	squashfuse-EXTERNAL
	GIT_REPOSITORY https://github.com/vasi/squashfuse/
	GIT_TAG 1f98030
	UPDATE_COMMAND "" # make sure CMake won't try to fetch updates unnecessarily and hence rebuild the dependency every time
	PATCH_COMMAND bash -xe ${CMAKE_CURRENT_BINARY_DIR}/patch-squashfuse.sh
	CONFIGURE_COMMAND ${LIBTOOLIZE} --force
	COMMAND env ACLOCAL_FLAGS="-I /usr/share/aclocal" aclocal
	COMMAND ${AUTOHEADER}
	COMMAND ${AUTOMAKE} --force-missing --add-missing
	COMMAND ${AUTORECONF} -fi || true
	COMMAND ${SED} -i "/PKG_CHECK_MODULES.*/,/,:./d" configure # https://github.com/vasi/squashfuse/issues/12
	COMMAND ${SED} -i "s/typedef off_t sqfs_off_t/typedef int64_t sqfs_off_t/g" common.h # off_t's size might differ, see https://stackoverflow.com/a/9073762
	COMMAND CC=${CC} CXX=${CXX} CFLAGS=${CFLAGS} LDFLAGS=${LDFLAGS} <SOURCE_DIR>/configure --disable-demo --disable-high-level --without-lzo --without-lz4 --prefix=<INSTALL_DIR> --libdir=<INSTALL_DIR>/lib --with-xz=${xz_PREFIX} ${EXTRA_CONFIGURE_FLAGS}
	COMMAND ${SED} -i "s|XZ_LIBS = -llzma |XZ_LIBS = -Bstatic ${xz_LIBRARIES}/|g" Makefile
	BUILD_COMMAND ${MAKE}
	BUILD_IN_SOURCE ON
	INSTALL_COMMAND ${MAKE} install
	UPDATE_DISCONNECTED On
	)

and

libappimage/ci/install-deps.sh

Line 56 in e56c21b

liblzma-dev

It seems that @TheAssassin had invested a lot of time to think about internal vs. external building (rather than my typical style of coding "linearly" to make it work in exactly one "known good" configuration), so I'll leave it up to him to add zstd support in order not to inadvertently break this flexibility.