ImageView doesn't shrink images to fit

[freakboy3742]

Describe the bug

The documentation for ImageView says that a flex=1 image will be expanded or shrunk to fit the available space. It definitely expands images to fit, but a large image will not be shrunk to fit.

Steps to reproduce

Use the following app code:

        main_box = toga.Box(style=Pack(direction=ROW))

        main_box.add(toga.ImageView("image.png", style=Pack(flex=1)))

        self.main_window = toga.MainWindow(title=self.formal_name)
        self.main_window.content = main_box
        self.main_window.show()

if image.png is smaller than the window, it will be expanded; if image.py is larger than the window, the window will be expanded to fit the natural size of the window (or, on mobile, show "content doesn't fit window" warnings).

Expected behavior

An ImageView with flex=1 should shrink large images to fit the available space, preserving aspect ratio.

Screenshots

No response

Environment

• Operating System: all (tested on macOS and iOS)
• Python version: All
• Software versions:
  • Toga: 0.4.0

Logs

No response

Additional context

The underlying problem appears to be that the intrinsic constraint has been set to at_least(size) in each axis, rather than at_least(0). This enforces a minimum size that can't be shrunk.