Redraw the macOS window only when something changed, and cache scaled pictures - #898

Fixes the macOS repaint item from todo.md. The binaries were rebuilt by build-native (#897, fast-forwarded into this branch).

• Redraw only on change. Runtime.present marks the view for display only when the frame differs from the last one (Frame is now Equatable) or when a picture it names was rewritten, appeared or went (Renderer.picturesChanged, a stat per shown picture). The managed loop presents at 60 fps, and every present redrew the whole window, so an idle viewer kept a core busy. Resizes and splitter drags were already invalidated by AppKit and the view. A move to a display of another scale now invalidates too (viewDidChangeBackingProperties), because it changes the device pixels a scaled picture fills.
• Cached scaled pictures. A picture drawn smaller than its own size is kept scaled to the device pixels it fills (Renderer.fitted), and a redraw copies it rather than resampling the source. The cache is dropped with the picture, so the existing eviction still bounds it. A picture drawn at or above its own size is drawn as before, which is every picture the capture baselines contain, so they're unaffected.

Not measured: there's no Mac here, so CI compiling the Swift and running the macOS pixel tests is the verification. Watching Activity Monitor on an idle viewer before and after would show the effect.

Only native/swift and todo.md change. The Windows image perf item, which #896 fixes, is still listed in this branch's todo.md and will merge cleanly beside it.