VideoConfig: Make StereoMode an enum class

Makes for more strongly-typed identifiers (and doesn't pollute
surrounding namespaces)

Source/Core/VideoBackends/OGL/FramebufferManager.cpp

@@ -133,7 +133,7 @@ FramebufferManager::FramebufferManager(int targetWidth, int targetHeight, int ms
 
   glActiveTexture(GL_TEXTURE9);
 
-  m_EFBLayers = (g_ActiveConfig.iStereoMode > 0) ? 2 : 1;
+  m_EFBLayers = (g_ActiveConfig.stereo_mode != StereoMode::Off) ? 2 : 1;
   m_efbFramebuffer.resize(m_EFBLayers);
   m_resolvedFramebuffer.resize(m_EFBLayers);