The textLayer does not update correctly when zooming with USE_ONLY_CSS_ZOOM = true

[Snuffleupagus]

When zooming with USE_ONLY_CSS_ZOOM = true, the textLayer doesn't update correctly anymore.
Open: http://mozilla.github.io/pdf.js/web/viewer.html#useOnlyCssZoom=true&textLayer=visible, notice how the textLayer overflows the page boundaries when zooming.

This is a regression from 5bd8a83, i.e. PR #4570.

Screenshots of the effect:

• When the viewer is initially loaded
  
• After zooming (the actual zoom level seem irrelevant)
  

[timvandermeij]

Just adding that it only happens for pages that are already rendered. If you scroll down to pages that are not rendered yet, the text layer with zoom is fine.