Additional parsing stages in `ShaderLab`

[Sway007]

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Additional parsing stages in ShaderLab to achieve on-demand compilation.

graph TD;
    Shader
    SubShader
    ShaderPass
    GLSL
    Shader-- "(Parser1)\nRenderState/Tag\nin SubShader scope,\nkeep other un-compiled text" -->SubShader;
    SubShader-- (Parser2)\nRenderState/Tag\nin ShaderPass scope,\nkeep other un-compiled text -->ShaderPass;
    ShaderPass --(Parser3)--> GLSL

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Current parsing flow

graph TD;
    Shader
    ShaderPass
    GLSL
    Shader-- "(Parser1)\nRenderState/Tag\nin SubShader and ShaderPass scope,\nkeep other un-compiled text" -->ShaderPass;
    ShaderPass --(Parser2)--> GLSL

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