Fix failing TransformFeedback GL test on Mesa.

src/Magnum/Test/TransformFeedbackGLTest.cpp

@@ -434,6 +434,9 @@ void TransformFeedbackGLTest::interleaved() {
                 "void main() {\n"
                 "    output1 = inputData + vec2(1.0, -1.0);\n"
                 "    output2 = inputData.x - inputData.y + 5.0;\n"
+                /* Mesa drivers complain that vertex shader doesn't write to
+                   gl_Position otherwise */
+                "    gl_Position = vec4(1.0);\n"
                 "}\n").compile());
             attachShader(vert);
             bindAttributeLocation(Input::Location, "inputData");