Tweak IME selection background left bound

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.

whoops

Fix warnings when calculating image color

This fixes warnings about narrowing and missing a component of the
struct.