auto-preview leaves buffers open

[jkassismz]

as i auto-preview files, my open-buffer list grows out of control. the preview function should open a buffer to preview a file and then close it when the next file is previewed. if the user navigates away from the quickfix window, the buffer become "permanent". but if the user quits the quickfix window, the buffer disappears and the user sees the buffer they saw before the search began.