Smaller PostEffect refactor and simplification, WebGPU support

[mvaligursky]

• the vertex shader all posteffects use is moved to the base class to avoid duplication, and adjusted to handle WebGPU y-flip.
• standalone pc.drawFullscreenQuad function has been deprecated, and a simpler member function has been added instead: drawQuad(target, shader, rect), which uses existing drawQuadWithShader that supports WebGPU
• PostEffect.shader has been removed, as it's not used by the class.
• post effect shader is created using pc. pc.createShaderFromCode instead of new pc.Shader, which handles platforms differences and allows it to run on WebGPU
• posteffect-outline script has been adjusted to work with this as a test. When this PR gets approved, other post effects would be adjusted too.

This makes the ModelOutline example work on the WebGPU (without omni shadows for now)