Medialib: album art requests may be implemented inefficiently, and may require some throttling.

[Oleksiy-Yakovenko]

Steps to reproduce the problem

• Open medialib
• Trigger loading album art for many albums
• Observe that loading is slower than it should be / fires too many requests

What's going on? Describe the problem in as much detail as possible.

There's no certainty that there's a bug. Maybe just inefficiency.

Information about the software:

Deadbeef version: trunk OS: Linux / GTKUI