Also show the image properly.

CCMAIL:kimageshop@kde.org

Please, everyone who has a different gpu from me, build this branch
and test painting, zooming, panning, everything. Checkout the
krita-opengl2-rempt branch

We can now also start testing on windows and osx...

TODO:

* reintegrate OCIO (which was fragment-only)
* move the texture updating to a separate thread
* move the canvas rendering to a separate thread
* integrate with krita sketch, somehow
* reimplement the opengl cursor outline
* reimplement the gradient shaders (perhaps? Not sure how useful they were.)
* reimplement the 3d cursors (low prio -- they were fun, but I guess no
pro artist used them)
* figure out a way to move the layerstack composition to the GPU?