In GimpViewRendererDrawable, use
gimp_drawable_get_sub_preview_async(), added in the previous
commit, to render drawable previews asynchronously.  While the
preview is being rendered, either keep showing the previous
preview, or render a placeholder icon.

This commit also fixes an issue where, under certain conditions, a
drawable preview would be rendered, even when layer/channel
previews are disabled in the preferences.

(cherry picked from commit 8a81bfd2)