Invert fixed color cursor if it's close to cell bg

This should reduce the number of times people with fixed cursor colors
run into troubles when existing text is already colored.

Using just the background color as a metric instead of both background
and foreground color should ensure that the cursor still has a clear
shape, since just changing the foreground color for a cursor might be
difficult to see. Always inverting the entire cursor instead of keeping
the fixed foreground color is important to make sure the contrast isn't
messed up.

Fixes #4016.

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