Enable adding capability: MultiViewport

Fix issue #191:
https://github.com/KhronosGroup/glslang/issues/191

SPIRV/GlslangToSpv.cpp

@@ -366,7 +366,7 @@ spv::BuiltIn TGlslangToSpvTraverser::TranslateBuiltInDecoration(glslang::TBuiltI
         return spv::BuiltInCullDistance;
 
     case glslang::EbvViewportIndex:
-        // TODO: builder.addCapability(spv::CapabilityMultiViewport);
+        builder.addCapability(spv::CapabilityMultiViewport);
         return spv::BuiltInViewportIndex;
 
     case glslang::EbvSampleId: