Examples: Improve AA approach in WebGPU water demo.

[Mugen87]

Related issue: a9666c6

Description

Unfortunately, enabling MSAA with a MRT setup somewhat kills the performance on mobile. On a Pixel 8a, webgpu_water stutters quite noticeably.

The PR introduces a more lightweight FXAA which produces still good results but without the overhead of multisampling.

https://rawcdn.githack.com/Mugen87/three.js/ce436cda9e1dd19f77172d30dcb502fe03f7492b/examples/webgpu_water.html
Prod for comparison: https://threejs.org/examples/webgpu_water

[Mugen87]

FXAA also produces better results than MSAA when looking flat on the textured surface:

MSAA:

FXAA:

I understand that MSAA works better in other places but for this particular scene FXAA seems to produce good results with way better performance.

Until traaPass() is compatible with mrt() (see #29842), I recommend to use smaa() or fxaa() for MRT FX setups at the end of the pass chain. At least if the app is supposed to run on mobile devices.

[sunag]

Until traaPass() is compatible with mrt()

I'll take a look at that...