There was a double loop when drawing the offscreen data for the fill tool. This loop did an exponential drawing, slowing a lot the process.
For example, one file was using 7.39 seconds and now use 1.20 or less to do the same process.
The fix is more noticeable with big files, reducing exponentially the time used by the tool. For very simple files the speed change can be unnoticeable.
Aso fixed a hidden bug with stroke opacity that it was unnoticeable due the double loop.
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