atlas: display underlines in the right color when they're highlighted

[DHowett]

This pull request resolves a longstanding issue (which probably nobody
but me cared about :P) where gridlines and underlines did not change
color to match the selection or search highlight foreground.

During brush preparation, we stash the underline color; during line
rendering we store it in a third bitmap and during rendering in each
backend we read the colors out of the appropriate bitmap.

The gridlines come from the foreground bitmap and the underlines from
the new underline bitmap.

I've updated each instance of append*Line to break the line-to-render up
by color span.

We are, therefore, no longer passing the stroke color in each gridline
payload.

The original console renderer supports painting the gridlines in
different colors for each half of a 2-cell glyph. That functionality no
longer works in Atlas. To fix it, we would need to move underline span
preparation into PaintBufferLine (so that we could apply the right
colors to the bitmap mid-cell.)

Known Issues

• D2D only; dotted lines which are broken by a highlight have doubled
  dots (because we draw them as individual line segments and without a
  global geometry like curly lines.)
• Atlas (all); grid line colors can no longer change in the middle of
  a 2-cell glyph.

Tested By

RenderingTests