Modernize legacy posteffect scripts to ES6 classes and fix FXAA offset bug

[mvaligursky]

Modernize legacy posteffect scripts to ES6 classes and fix FXAA offset bug

Fixes #5218

Updates all legacy posteffect scripts in scripts/posteffects/ from ES5 prototype-based inheritance to modern ES6 class syntax, and fixes a long-standing FXAA texture offset bug.

Changes

ES6 Class Modernization (all 14 posteffect files)

• Convert function constructors to ES6 class extends pc.PostEffect
• Replace pc.PostEffect.call(this, graphicsDevice) with super(graphicsDevice)
• Remove prototype boilerplate (Object.create, Object.assign, etc.)
• Convert methods to class method syntax
• Update var declarations to const/let
• Convert shader strings from [...].join('\n') to template literals with /* glsl */ tag for editor syntax highlighting

FXAA Fix

• Fixed texture offset bug where corner samples were incorrectly offset by √2 pixels instead of 1 texel
• Use WebGL2 textureOffset() for proper 1-texel sampling and better GPU performance

Updated Files

• posteffect-blend.js
• posteffect-bloom.js
• posteffect-bokeh.js
• posteffect-brightnesscontrast.js
• posteffect-edgedetect.js
• posteffect-fxaa.js
• posteffect-horizontaltiltshift.js
• posteffect-huesaturation.js
• posteffect-luminosity.js
• posteffect-outline.js
• posteffect-sepia.js
• posteffect-ssao.js
• posteffect-verticaltiltshift.js
• posteffect-vignette.js