Make it possible to call grabToImage() to get a 16bit based (64bpp) image back

Make it possible to call QQuickItem::grabToImage() to get a 16bit based (64bpp) image back. Currently it will give an 8bit image (32bpp) back because the internal texture format is GL_RGBA, whereas to get a 16bit one back we would need it to be GL_RGBA16. This can be necessary now we have support for 16bit based images.