Simplify gimp_view_renderer_drawable_render(), by consolidating
common code paths.  In particular, when rendering the preview as
part of an image, always crop the preview to the bounds of the
image, even when downscaling, to avoid unnecessarily downscaling/
convering cropped-out regions.  We previously only did this when
upscaling the preview by a factor of 2 or more; whatever the reason
for this used to be, it's no longer there.