Explicitly add z-index: 0 to .textLayer

This creates a "stacking context", making sure that the elements
inside `.textLayer` (on the z axis) remain inside `.textLayer`.
Without this change, the `<span>`s inside `.textLayer` end up being
on top of `.annotationLayer`.

This was caught by the freetext editor integration tests, that
failed because we were trying to draw on top of some text.

web/text_layer_builder.css

@@ -24,6 +24,7 @@
  forced-color-adjust: none;
  transform-origin: 0 0;
  caret-color: CanvasText;
+ z-index: 0;
 
  &.highlighting {
  touch-action: none;