Allow older cmake versions (Maintenance release)

No code changes since 1.4.0, just updates to cmake files (being able to work with older cmake versions) and documentation.

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 )