This RR is based on 4 commits:
* This first patch allows us to track the GLTexture wrapmode internally.
* The second patch improves the texture update handling:
  It changes the behavior of strictly bound textures such that
  they are only updated if the corresponding window has been damaged.
  Additionally GLTexture now keeps track of the current filter setting.
* The third patch avoids unnecessary texture binds within performPaint.
* The last patch cleans up the GLTexture API.

nvidia blob works with QT_GRAPHICSSYSTEM=raster/native and GL2/GL1.
Would be nice if somebody could test it with hardware which really requires the strict texture binding.