gl_rasterizer: Allow rendering without fragment shader

Rendering without a fragment shader is usually used in depth-only
passes.

src/video_core/renderer_opengl/gl_shader_manager.h

@@ -50,6 +50,10 @@ public:
         current_state.geometry_shader = 0;
     }
 
+    void UseTrivialFragmentShader() {
+        current_state.fragment_shader = 0;
+    }
+
 private:
     struct PipelineState {
         bool operator==(const PipelineState& rhs) const {