Create stacking context. fix #39 #43

[ghetolay]

When text is rendered canvas has a z-index of 1 to be kept under the text.
But because there is no stacking context anywhere inside <pdf-viewer> to encapsulate z-index, this can provoke unexpected behavior with user designs like in #39 and #43.

This is one possible fix, another one could be to set the text layer as position: absolute instead of the canvas and keep the default z-index behavior.

Workaround in the meantime :

 pdf-viewer {
   position: relative;
   z-index: 0;
 }