app: simplify drawable preview rendering
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.
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