Update CodingGuide with standard styling for compiler directives

[jjhembd]

Based on some discussion in #11970:
Some of our shader code can be difficult to read due to deeply nested compiler directives. The readability problem is exacerbated by inconsistent indentation. For example, in MaterialStageFS, we had:

void materialStage(...)
{
    // ...
    #if defined(LIGHTING_PBR) && defined(USE_SPECULAR_GLOSSINESS)
        #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE
        vec2 specularGlossinessTexCoords = TEXCOORD_SPECULAR_GLOSSINESS;
          #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE_TRANSFORM
          specularGlossinessTexCoords = computeTextureTransform(specularGlossinessTexCoords, u_specularGlossinessTextureTransform);
          #endif

        vec4 specularGlossiness = czm_srgbToLinear(texture(u_specularGlossinessTexture, specularGlossinessTexCoords));
        vec3 specular = specularGlossiness.rgb;
        float glossiness = specularGlossiness.a;
            #ifdef HAS_SPECULAR_FACTOR
            specular *= u_specularFactor;
            #endif
// ...

One popular style guide recommends that all preprocessor directives start at the beginning of the line, and that the directives should not affect the indentation of the rest of the code. This would be a significant change to all our shaders.

Another strategy is to reduce the level of nesting by pulling some of the #define logic out into a subroutine. The above sample then becomes:

void setSpecularGlossiness(inout czm_modelMaterial material)
{
    #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE
        vec2 specularGlossinessTexCoords = TEXCOORD_SPECULAR_GLOSSINESS;
        #ifdef HAS_SPECULAR_GLOSSINESS_TEXTURE_TRANSFORM
            specularGlossinessTexCoords = computeTextureTransform(specularGlossinessTexCoords, u_specularGlossinessTextureTransform);
        #endif

        vec4 specularGlossiness = czm_srgbToLinear(texture(u_specularGlossinessTexture, specularGlossinessTexCoords));
        vec3 specular = specularGlossiness.rgb;
        float glossiness = specularGlossiness.a;
        #ifdef HAS_LEGACY_SPECULAR_FACTOR
            specular *= u_legacySpecularFactor;
        #endif
//...
void materialStage(...)
{
    // ...
    #if defined(LIGHTING_PBR) && defined(USE_SPECULAR_GLOSSINESS)
        setSpecularGlossiness(material);
    #elif //...

Reduced nesting would be readability improvement, even independent of the choice we make for indentation.

Options for the CodingGuide could be:

• Recommend subroutines any time compiler directives exceed 3 levels of nesting
• Choose an indentation strategy, and find/configure a glsl linter to automate it