HiDPI support

## Lore

issues #207 and #236 were about a **bug** happening with HiDPI screens, causing data loss, and a few unusable tools. These bugs have been closed by 7047724ce27569847dfcde7f541f10f62e1fd800 where i hardcoded a device scale factor of 1.0 on all cairo surfaces that seem to need it to avoid the bug.

The actual bug was, in my opinion, that scaling was applied several times to a few of these surfaces. 1.0 × 1.0 = 1.0, thus the issue is mitigated. But not really fixed

## Requested feature

Instead of hardcoding the factor at 1.0 everywhere, it should be 1.0 only where scaling is applied twice, and elsewhere it should be the value set by the toolkit

## Use case

The goal is to have big enough images on the huge displays, because a pic at scale 1.0 is comically tiny if everything else is at 200% (and not very usable as an editor, even if the zoom can now help)

No files, tests, or entry points are named. Start by tracing the Cairo surfaces that currently hardcode a device scale factor of 1.0, compare them with the toolkit-provided scale, and verify that scaling is applied only once while HiDPI images remain usable.

Written by the indexing model from the issue text.