Render selection fragments on opaque lines

Because lines are opaque on the GPU for sub pixel antialiasing, the
lines obscure the selections which were formerly rendered behind the
lines. This commit renders selection fragments *on* each opaque line
layer so the selections look correct again. Still needs cleanup and
optimization.
Make multi-line selections appear to span the screen with a single div

Because lines are opaque and any area of a selection that overlaps a
line is actually rendered on the line itself, the screen-spanning
background of a multi-line selection can actually be rendered as a
single div spanning the entire screen from the first row to the
penultimate row of the selection.