OpenGL rendering in RasterImageView

Beginning of OpenGL rendering in gwenview. Current state:
if OpenGL rendering mode is selected the image is rendered
through an OpenGL shader. The scaling of pixmaps is no longer
needed.

There are still some areas to improve:
* Better detection of OpenGL availability
* Better detection whether OpenGL is working (creation shaders)
* Some regressions like compare mode not working
* Updating the texture on changes