gl_shader_gen: AppendAlphaModifier default should be 0.0, not vec4(0.0).

src/video_core/renderer_opengl/gl_shader_gen.cpp

@@ -163,7 +163,7 @@ static void AppendAlphaModifier(std::string& out, TevStageConfig::AlphaModifier
         out += ".b";
         break;
     default:
-        out += "vec3(0.0)";
+        out += "0.0";
         LOG_CRITICAL(Render_OpenGL, "Unknown alpha modifier op %u", modifier);
         break;
     }