SpaceDrawer: Avoid repeatedly checking last line
SpaceDrawer: Avoid repeatedly checking last line

The iteration logic for drawing spaces doesn't take into account the
last line in the file, such that the last line always iterates across
the entire length of the line, performing expensive GtkTextIter
operations, even though it's already progressed past the end of the
visible line. This causes problems on files with very long lines as the
last line in the file, which is common on e.g., normal UTF-8 that's been
incorrectly decoded as UTF-16.

The fix here just checks to see whether the forward-to-next-line case
actually forwards to a new line.

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