VideoConfig: Add StereoMode enumeration.

Source/Core/VideoBackends/OGL/Render.cpp

@@ -1514,7 +1514,7 @@ void Renderer::SwapImpl(u32 xfbAddr, u32 fbWidth, u32 fbStride, u32 fbHeight, co
 
 			sourceRc.right -= fbStride - fbWidth;
 
-			if (g_ActiveConfig.iStereoMode == 1)
+			if (g_ActiveConfig.iStereoMode == STEREO_SBS)
 			{
 				// Resize target to half its original size
 				int width = drawRc.GetWidth();
@@ -1544,7 +1544,7 @@ void Renderer::SwapImpl(u32 xfbAddr, u32 fbWidth, u32 fbStride, u32 fbHeight, co
 		// for msaa mode, we must resolve the efb content to non-msaa
 		GLuint tex = FramebufferManager::ResolveAndGetRenderTarget(rc);
 
-		if (g_ActiveConfig.iStereoMode == 1)
+		if (g_ActiveConfig.iStereoMode == STEREO_SBS)
 		{
 			// Resize target to half its original size
 			int width = flipped_trc.GetWidth();

Source/Core/VideoCommon/VideoConfig.h

@@ -44,6 +44,12 @@ enum EFBScale
 	SCALE_4X,
 };
 
+enum StereoMode
+{
+	STEREO_OFF = 0,
+	STEREO_SBS
+};
+
 // NEVER inherit from this class.
 struct VideoConfig final
 {