Fix compilation of transform feedback test shaders on Mesa.

Apparently they need to write something to gl_Position.

src/Magnum/Test/PrimitiveQueryGLTest.cpp

@@ -163,6 +163,9 @@ void PrimitiveQueryGLTest::transformFeedbackPrimitivesWritten() {
                 "out mediump vec2 outputData;\n"
                 "void main() {\n"
                 "    outputData = vec2(1.0, -1.0);\n"
+                /* Mesa drivers complain that vertex shader doesn't write to
+                   gl_Position otherwise */
+                "    gl_Position = vec4(1.0);\n"
                 "}\n").compile());
             #ifndef MAGNUM_TARGET_GLES
             attachShader(vert);

src/Magnum/Test/TransformFeedbackGLTest.cpp

@@ -199,6 +199,9 @@ XfbShader::XfbShader() {
         "out mediump vec2 outputData;\n"
         "void main() {\n"
         "    outputData = inputData + vec2(1.0, -1.0);\n"
+        /* Mesa drivers complain that vertex shader doesn't write to
+           gl_Position otherwise */
+        "    gl_Position = vec4(1.0);\n"
         "}\n").compile());
     #ifndef MAGNUM_TARGET_GLES
     attachShader(vert);
@@ -308,6 +311,9 @@ XfbMultiShader::XfbMultiShader() {
         "void main() {\n"
         "    output1 = inputData + vec2(1.0, -1.0);\n"
         "    output2 = inputData.x - inputData.y;\n"
+        /* Mesa drivers complain that vertex shader doesn't write to
+           gl_Position otherwise */
+        "    gl_Position = vec4(1.0);\n"
         "}\n").compile());
     #ifndef MAGNUM_TARGET_GLES
     attachShader(vert);