VideoCommon: Add a separate constants buffer for the geometry shader.

Source/Core/VideoCommon/ConstantManager.h

@@ -43,6 +43,9 @@ struct VertexShaderConstants
 	float4 normalmatrices[32];
 	float4 posttransformmatrices[64];
 	float4 pixelcentercorrection;
-	float4 stereoparams;
 };
 
+struct GeometryShaderConstants
+{
+	float4 stereoparams;
+};