GL2PaintEngine: centralize logic for updating/preparing textures

Rewrite of updateTextureFilter to include activating and binding as well,
so that we can maintain a single view of which texture was the last one
to be used from within the engine itself.

The behavior should be the same as before.

Change-Id: I41781d00458b0176c614266f4360db3c68b120a1
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>