As mentioned in e072853e63, fb6ac24514 and 9a6beb915d,
`file_draw_preview()` is a rather overloaded and confusing function. I'm
trying to make it more readable.
This splits off the drawing for the loading icon displayed while the
previews are pending/loading still, removing the loading case handling
from `file_draw_preview()`. There was also some implicit logic here:
While loading previews we'd always pass a "special image" to the preview
drawing, so the `is_special_file_image` boolean would always be true.
This is untangled too now, so code paths are more explicit/clear.
`file_add_preview_drag_but()` can't access data returned by
`file_draw_preview()` anymore (it may not be called), so I made it
independent which is an improvement too.
While working on this I noticed the loading icon isn't centered
correctly. For now I made sure the position remains the same, I'll fix
the positioning in a followup.
cd13f52e75