Fix unexpected mute when window becomes focused while fast-forwarding

[ahigerd]

Reported by user "Vapoure" in Discord.

Steps to reproduce

1. In Settings -> Audio/Video -> Fast Forward, check Mute
2. Uncheck Mute
3. Ensure that none of the "Pause" or "Mute" options are checked in the Interface tab
4. Start a game that will produce audio
5. Toggle fast-forward (shift+tab or menu option)
6. Switch to another window
7. Switch back to the mGBA window

Expected results

There should be no audio interruptions, because none of the Pause or Mute options are turned on.

Actual results

The audio becomes muted when the window regains focus.

Additional details

It turns out that there's a difference between the default behavior of the Fast Forward Mute option and the "off" behavior -- this issue cannot be reproduced if the fastForwardMute line in config.ini does not exist. I'm not sure of the historical background for why this option's unconfigured behavior would be expected to differ from either the checked or unchecked behavior, but the logic for working with the -1 value is clearly mistaken.

I also separated the "inactive" and "minimized" boolean flags because there's a potential edge case that the window could receive a showEvent without gaining the focus, which would result in unexpected behavior. (This is possible with some window managers.)

[ahigerd]

This appears to be related to #3547 and might fix that problem as well.