Add OpenSlide support

Thanks to @lromor , we now have OpenSlide support showing the overview of multi-resolution images. This will help biologists and astronomers alike to quickly scan through their imagery.

Usability fix: if multiple files given, only loop animations once

This is mostly fixing a regression that used to work but broke due to a typo.

Bugfix release: don't loop static PNG images processed by video-decoder

Changes

• Non-animated PNG files that happen to be handled by the video decoder instead of the image decoder (e.g. due to explicitly chosen -V or because they have been fetched from an URL), should not be considered a 'loopable' animation unless they actually have more than one frame.
• Minor: Make the synopsis fit into 80 character-wide terminals.

Better timing on slow machines; 8 bit color mode

• Write buffers to terminal asynchronously to reduce latency on slow machines for more accurate timing of animations and movies.
• Provide --color8 fallback mode for terminals not doing 24 bits color (but, you should still upgrade to a modern terminal emulator :) ).

Improve auto-detecting of terminals

Update release.

• Feedback from iTerm2 user revealed that the background color response is slightly different from other terminals.
• Kitty graphics protocol query sometimes messes up other terminals (Konsole, iTerm2). So for now, just look at $TERM.
• Increase some timeouts for terminal queries to accommodate remotely logged in sessions.
• Less CPU use for --compress

Most notable: iTerm2 graphics

• Most notable addition in this release is to support iTerm2 graphics protocol. Tested with wezterm (if you have a Mac, please test with an iTerm2). This allows to display high-resolution images in terminals that support iTerm2 graphics (known so far: iTerm2 itself, WezTerm and Chromium hterm). Mode is auto-detected for iTerm2 and wezterm).
• --compress works in Kitty and iTerm2 mode to PNG-compress the displayed images; good if ssh-bandwidth is limited.
• A few smallish bugs fixed since last release.

Kitty graphics, quarter blocks and improved transparency

• If runnning in a kitty terminal, uses the kitty graphics protocol to output high-resolution images
• Improved handling of transparency, best blending in with your terminals' background and improved alpha blending.
• In animations and movies, only update parts of screen that actually changed (reduces load on the terminal emulator)
• Additional pixelation mode (quarter blocks) that trades improved spatial resolution in x-direction with slightly less color accuracy.
• Usability improvements here and there.

Fix -f to resolve relative filenames to listfile; improve JPEG loading

• Reading a list of images to show via -f resolved filenames in there relative to the current working directory instead of relative to the list file.
• Special case JPEG loading as we can make use of down-sampling while decoding, which is not a feature offered by GraphicsMagick.

Provide grid display, threaded loading, long options, ...

• The new option --grid now allows to arrange pictures in a grid on the screen. Useful if displaying a large number of images.
• Threads: image loading can be slow, so they're now loaded in multiple threads (by default using half the reported cores)
• Many features now can also be selected with long options, as a huge amount of short ones starts to get cryptic. Some options got removed or renamed during the last development phase as a base to be more stable in the future.
• Added option -f (loading a list of images to display from a file) and -o (output to specific file)

Also:

• Changed build system to CMake which makes it easier to port between systems (thanks @coldtobi )
• Convert man-page to using pandoc, provide build instructions for macOS as well as establishing github actions for Continuous Integration (thanks @speedy-beaver )

Play videos, more options

Most notable in this release are

• timg can now also play videos (compile-time choice).
• A manpage now makes this a 'proper' tool.
• A few more options relevant for video playing as well as centering images with -C or switching off antialiasing with -a. Probably a few more I forgot.
• The character to use to display pixels can now be changed via an environment variable.
• Some silly performance improvements (so we could show 3000fps if the terminal would be able to deal with it...).