Draft: Add Blurring mode to Smudge Engine

This adds a new 'Blurring' mode to the 'Smudge Engine'. The blurring mode uses a Gaussian blur. This lets us paint really sexy smoothing, and replicate a substantial part of the functionality of SAI's watercolor brush.

Additionally, a 'Smear offset' flag is added to toggle whether to use the previous brush location or the current brush location as source. This adds a variation to both the Dulling and Blurring modes that appears more localized.

2022-05-28_22-48-45

Test Plan

Set 'Smudge mode' to 'Blurring', the smudge will now be blurred. Turn off 'Smear offset', the blur will remain more localized.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.