Convert/package faster!

[nirvn]

This pull request dramatically (maybe I'm being a bit too... dramatic 😉 ) increase packaging speed in large projects (or projects containing slow remote data sources) by avoiding the cost of a full project load when merely wanting to fetch the map canvas XML.

In addition, a new convert() function parameter allows skipping of a final project reload when not needed (i.e., when packaging projects on the cloud).

[suricactus]

Big +1 for the QgsProject.read() flags approach. However, I am not convinced on the temporary QgsProject and file approach. Can you elaborate what you are trying to achieve by introducing them?

[nirvn]

@suricactus , as mentioned in my reply to your comment above, the introduction of a temporary QgsProject is done so we can use the QgsProject.FlagDontResolveLayers , which makes project loading blazingly fast. Since we're only looking for the map canvas XML element, we can make use of that flag and say bye bye to long project load.

If we would be doing this on the main QgsProject.instance() singleton, we would be unable to then use the project to manipulate layers.

[suricactus]

Thanks for the explanations. Can we just put them in the code as I attempted in #69 . I tried putting my understanding there.

[nirvn]

@suricactus , good idea. You want to push your commit into this PR or we merge both sequentially?

[suricactus]

If you agree with my proposed changes in #69, when you merge, they will come into this PR. On my PR I set the base branch to be your branch here.