kwineffects: Fix potential image detaching
QImage is implicitly shared. However, methods such as QImage::bits() may detach. In order to avoid the detach, one either needs make the image const or use QImage::constBits() method.
Currently, the GLTexture class uses QImage::bits() all over the place. It prevents implicit detaching by storing images in const variables. Such code is fragile. Adding an intermediate (optional) conversion step may result in odd performance regressions.