Unbind the event handler referencing a local variable, as leaving it
bound could result in a crash later if another paint event was generated
for the window for whatever reason.
Doing it like this requires using 2 different objects, but the
complexity can be still hidden inside WaitForPaint class, with the 2nd
object being just a member of it, and, in fact, makes the code using it
simpler as it doesn't need to use a boolean variable with it.
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