TextLayer selection coloring improvement for overlap.

As a consequence of merging #5221 it is more likely to have multiple
overlapping selection divs inside the text layer. Because each individual
element gets the selection style applied, the 30%opacity stacks, making a
60% bar visible where the overlap happens.

As proposed by @rocallahan, this can be fixed by making the selection
style solid and setting opacity for the overall layer.

I assume also that this should make the work for the renderer easier, but
was unable to bench it.
Merge pull request #5282 from CodingFabian/nicer-overlapping-text-selection

TextLayer selection coloring improvement for overlap.