gl_rasterizer: Signal UNIMPLEMENTED when rt_separate_frag_data is not zero

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -430,7 +430,7 @@ void RasterizerOpenGL::ConfigureFramebuffers(OpenGLState& current_state, bool us
     // TODO(bunnei): Figure out how the below register works. According to envytools, this should be
     // used to enable multiple render targets. However, it is left unset on all games that I have
     // tested.
-    ASSERT_MSG(regs.rt_separate_frag_data == 0, "Unimplemented");
+    UNIMPLEMENTED_IF(regs.rt_separate_frag_data != 0);
 
     // Bind the framebuffer surfaces
     current_state.draw.draw_framebuffer = framebuffer.handle;