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When painting a line, the multidata of the line /before/ it is valid
in most cases: it was determined just a moment ago.  And it tells us
all we need to know: whether there is an unpaired start match before
the current line or not.

The only exception is when painting the first line of the screen:
the multidata of the line before it might be stale.  So for the
first screen line we will always have to do some backtracking.
But that is left for later.

This fixes https://savannah.gnu.org/bugs/?50121.