Just extending the selection background left by one pixel so there's
not one pixel of black background to the left of a selection that
starts at the beginning of the text, and so some characters being
selected show up better (particularly where there's a long vertical bar
at the first pixel). We shouldn't be overlapping any part of the
previous character, since every character normally has a pixel of
spacing on the right.
Decided to implement it anyway since the broken behavior (selection
length always being 0, at least on Windows) may get fixed later in SDL,
so let's do it right in one go.
This shows the uncommitted text in a box in the bottom left corner.
This doesn't show the selection (defined by the start and length fields
in the event) yet, but this is already much better than it was on its
own, and I don't know how urgent the selection is since it's broken on
Windows anyway.
