Fixup: Canvas No Longer Soft-Locks When Changing Graphics Acceleration Setting

Initial attempt to fix bug 423840 (Softlock when changing graphics acceleration), tracked origin of bug to commit: 5aef0e99 I plan to do more testing tomorrow, but wanted to post progress before I go to bed after a long bisect. :)

NOTE: Originally, we didn't always check for new canvas assignments, only that the canvas wasn't within a specific data structure. This is a potential solution, doesn't seem to introduce any major issues while using krita and allows the user to properly change graphics acceleration.

BUG:423840

Test Plan

@dkazakov It seems like the original commit was addressing another bug, we need to make sure that work isn't being undone here.

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