Make GL2PaintEngine::drawCachedGlyphs use updateTexture

Ensures that we have a consistent view of what the last used texture
was, which is critical when deciding whether or not we need to re-bind
the texture or update texture properties.

Change-Id: Ib47eb00abde98d148fc6e569ce3e359b340328fb
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>