Refactor stereo build flags and configs (#7857)

- Add option to build.sh to build for paritcular stereo
   techniques (default to NONE). Only applies to samples.
 - Consoldiate viewer checkbox for debugging stereo rendering
 - Add DriverConfig flag for stereoscopic type so that it can
   be used to determine availability of the feature and
   (to be completed) enable corresponding GPU features.

Co-authored-by: Mathias Agopian <mathias@google.com>

libs/filamat/src/GLSLPostProcessor.cpp

@@ -539,15 +539,16 @@ bool GLSLPostProcessor::fullOptimization(const TShader& tShader,
 
         if (config.variant.hasStereo() && config.shaderType == ShaderStage::VERTEX) {
             switch (config.materialInfo->stereoscopicType) {
-            case StereoscopicType::INSTANCED:
-                // Nothing to generate
-                break;
             case StereoscopicType::MULTIVIEW:
                 // For stereo variants using multiview feature, this generates the shader code below.
                 //   #extension GL_OVR_multiview2 : require
                 //   layout(num_views = 2) in;
                 glslOptions.ovr_multiview_view_count = config.materialInfo->stereoscopicEyeCount;
                 break;
+            case StereoscopicType::INSTANCED:
+            case StereoscopicType::NONE:
+                // Nothing to generate
+                break;
             }
         }