vo_opengl: remove pointless glFinish calls
player: print only changed tags

The code in the demuxer etc. was changed to update all metadata/tags at
once, instead of changing each metadata field. As a consequence,
printing of the tags to the terminal was also changed to print
everything on each change.

Some users didn't like this. Add a very primitive way to avoid printing
fields with the same value again if metadata is marked as changed. This
is not always correct (could print unchanged fields anyway), but usually
works.

(In general, a rather roundabout way to reflect a changed title with ICY
streaming...)

Fixes #813 (let's call it a "policy change").