Fix width calculation for individual line numbers

Since the gutter wrapper for each individual line number element has a `left` of `-totalGutterWidth` (see http://cl.ly/image/222Y1B1b0g1H), and the line number element needs to take up the whole width in the gutter wrapper, it should be `gutterTotalWidth - padding`, not `lineNumInnerWidth`.

Otherwise, if the main gutter element has borders or such (for instance, separating the gutter from the text), the line number element will be misaligned within its wrapper: http://cl.ly/image/3d3e101u2B1q