feat: render mode to pause rendering when window is blurred

[andretchen0]

As a developer using Tres, I would like a to have a render mode that pauses rendering when:

• the window/app containing the Tres Scene is blurred/backgrounded
• OR the tab/page containing the Tres Scene is blurred/backgrounded

And resumes rendering when:

• the window/app containing the Tres Scene is focused/foregrounded
• AND the tab/page containing the Tres Scene is focused/foregrounded

Context

@andretchen0 @alvarosabu In my tests with babylonjs (other engine but should be similar) this worked well, when the tab was in the background yes. But using multiple windows where the window running the engine is visible but not focused left it running at 60 FPS. Given many 3d websites/applications are very resource intensive I'd still consider adding the option to pause rendering when the tab/window is not focused.

Originally posted by @thomasaull in #497 (comment)