Move get_scanline_32/64 to the bits part of the image struct

Move get_scanline_32/64 to the bits part of the image struct

At this point these functions are basically a cache that the bits
image uses for its fetchers, so they can be moved to the bits image.

With the scanline getters only being initialized in the bits image,
the _pixman_image_get_scanline_generic_64 can be moved to
pixman-bits-image.c. That gets rid of the final user of
_pixman_image_get_scanline_32/64, so these can be deleted.