Metadata browser: misc improvements

This poor dialog was doing all of its rendering to an invisible picture
box, instead of using a buffer DIB.  (For shame.)

I would like to add metadata editing facilities at some later point in
6.6's development cycle, so I'm not dealing with finalizing this
dialog's layout just yet.  For now, however, this commit at least cleans
up the whole rendering process, while also fixing some annoyances with
the way the mousewheel was being handled.