Take negative left offset into account when forming SDF

[mourner]

From @ChrisLoer's #33 (comment):

You mentioned that the same logic for glyphWidth "doesn't affect rendering": I think that's just because the fonts we're using typically have an actualBoundingBoxLeft that's slightly negative? So even if the resulting glyphWidth is one pixel too small, the glyph doesn't look clipped from the right because it's actually eating slightly into the buffer on the left.

Right now we're counting on that as a common property of fonts, since we hardwire the x drawing position to this.buffer, and hardwire the left metric to 0. If we're worried, it would be straightforward to hook it up to the actual metrics the way we do for height.