Change compile flags from uint32_t to string key/value pairs

* For D3D this is overkill as we just stuff the uint32_t flags into a
  string. However for SPIR-V this will let us store the parameters from
  an OpModuleProcessed.

qrenderdoc/Windows/TextureViewer.cpp

@@ -3600,8 +3600,8 @@ void TextureViewer::reloadCustomShaders(const QString &filter)
           rdctype::str errors;
 
           ResourceId id;
-          std::tie(id, errors) =
-              r->BuildCustomShader("main", source.toUtf8().data(), 0, ShaderStage::Pixel);
+          std::tie(id, errors) = r->BuildCustomShader("main", source.toUtf8().data(),
+                                                      ShaderCompileFlags(), ShaderStage::Pixel);
 
           if(m_CustomShaderEditor.contains(key))
           {