Remove artifacts when merging multiple animated layers w/ layer styles.

Initial attempt to correct a bug where effects styles would leave artifacts when merging for animated content. Uploading for the sake of discussion w/ @dkazakov .

Test Plan

Download the test file from here and attempt to merge the multiple layers with layer styles enabled.

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.