Tiled Gallery: Fix carousel when grayscale set.

Images were added twice to carousel when grayscale was set to true, because carousel image arguments were set both to the actual image, and the grayscale overlay. If we don't set the attributes in grayscale images, the images are found in carousel only once.
Merge pull request #528 from rase-/fix/grayscale-carousel-interop

Tiled Gallery: Don't apply Carousel attributes to the grayscale images used in the non-hover state as they aren't shown in Carousel.