Change images to be marked as supported on iOS. This table probably needs some nuance attached though... but as a "quick glance of what's possible" it's fine for now.
Feature flags that clash are no fun. This reworks a few things so that
docs can properly build with all feature flags at once and note their
support.
Also adds a supported table to the README for ease of discovery.
- Changes internal target_os flags to be feature flags; macOS is now
appkit, and iOS/tvOS are now uikit. This enables platforms that are
not Apple-specific platforms that use frameworks to be compiled for.
- Updates the examples to handle closing/quitting better.
