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.