Proposal: Add an option to make device scale factor flexible, in addition to the existing --retina argument

[nielthiart]

The --retina argument is convenient to set a device scale factor of 2, but for my use case I needed a variety of scale factors to match other displays.

Ref: #133

For example, Google Pixel 6 has a scale factor of 2.625. Some devices go up to 4.

My proposal is to add an additional argument, --scale-factor, that takes a float as argument, and is None by default.

To avoid confusion when using --retina and --scale-factor simultaneously, we could treat them as mutually exclusive and raise a helpful error. We can also check that the scale factor is positive.

A reusable function that validates and normalises the device scale factor provides the opportunity to validate other scale-factor-altering arguments in the future, such as the --device argument proposed in #129.

[simonw]

From a successful GitHub Actions run:

simonwillison-retina:

simonwillison-scale-factor-pixel-six:

shot-scraper/tests/run_examples.sh

Lines 164 to 169 in b478542

	# --retina
	shot-scraper https://simonwillison.net/ -h 400 -w 800 \
	-o examples/simonwillison-retina.png --retina
	# --scale-factor
	shot-scraper https://simonwillison.net/ -h 915 -w 412 \
	-o examples/simonwillison-scale-factor-pixel-six.png --scale-factor 2.625