The render target is used to render the scene (or part of it) onto texture. This texture can then be used
e.g. to do some postprocessing.
Demo effect coming soon.
Move checkGLError() to kwineffects.*
Add GLTexture ctor which takes width and height and creates an empty texture (to be used with
GLRenderTarget to render onto it)
svn path=/branches/work/kwin_composite/; revision=655489