Coverart: don't draw until after image lookup, dynamic pixbuf cache, fix a GDK scaling issue

See how this looks.  I've changed coverart to only redraw once artwork lookup is complete.  Either successfully or failed, so the default artwork does not show until the artwork plugin has tried and failed to find an image (although that may be instant if the lookup has been tried before).  Most of the time this just means smoother image transitions, but try invalidating the cache and you'll see grey spaces for up to a few seconds.  I think it looks fine, but maybe not everyone will like it?  Note that albumart.org is extremely slow today, which can be interesting for testing.
I've also set the playlist pixbuf cache to resize dynamically.  This is primarily to prevent thrashing, which was rare but really bad when it happened.  Now it starts at 2 slots and increases as needed when cache thrashing is detected.  It never decreases.  It generally stabilises at around twice the number of images visible on the screen at one time.  For now, I've left in a diagnostic message showing when the cache size is increased.
Images smaller than 8 pixels are no longer drawn at all.