Add a debug message showing if GL acceleration if used for media playback

[jdm]

When investigating media playback issues, it can be tricky to remember all the variables that can affect if GL acceleration is in use or not. We should have a message that is always displayed that shows if GL is in use for playback or not.

[jdm]

Bonus points if there's some way of differentiating between possible reasons why it's not in use, like "GL media playback preference not enabled" or "media stack reports platform does not support GL playback".