[Unity] Performance improvement on SteamAudioManager.GetPerspectiveCorrection

[Chrisdbhr]

The problem

the method GetPerspectiveCorrection() in SteamAudioManager is called every LateUpdate() even when perspectiveCorrection is disabled in SteamAudioSettings and inside it there are multiple calls to Camera.main, this method searches for the first game object with the "MainCamera" tag, with can lead to performance issues on scenes with a lot of game objects, as shown bellow.

Solution implemented

• Do not search for camera when SteamAudioSettings.perspectiveCorrection is set to false, returning default values for the struct PerspectiveCorrection
• Cache the reference to the MainCamera when first found.

[lakulish]

There should probably be a NotifyMainCameraChanged function just like NotifyAudioListenerChanged, so that the cached Camera instance can be updated if it is explicitly changed by some script.