WebP: QImage detach in WebPPictureImportRGB(A) can cause a crash

Saving WebP detaches the QImage when using WebP API. `.bits()` is called instead of `.constBits()` which leads to worse performance when QImage is already in required format.

Moreover if we have custom (larger) bytes per line, after detaching the image will be copied to a smaller bytes per line (additional data padding removed), so it'll crash (the bytes per line is read and passed to WebP API before detaching).