GL: the unused member function warnings were useful, actually!

Because I had a stupid bug in the test and wasn't testing all of them.
And then the function itself had a bug that made it no-op exactly when
it shouldn't be a no-op.

src/Magnum/GL/AbstractShaderProgram.cpp

@@ -524,7 +524,7 @@ AbstractShaderProgram& AbstractShaderProgram::drawTransformFeedback(Mesh& mesh,
 
 AbstractShaderProgram& AbstractShaderProgram::drawTransformFeedback(MeshView& mesh, TransformFeedback& xfb, UnsignedInt stream) {
     /* If nothing to draw, exit without touching any state */
-    if(mesh._instanceCount)
+    if(!mesh._instanceCount)
         return *this;
 
     use();

src/Magnum/GL/Test/AbstractShaderProgramGLTest.cpp

@@ -1319,11 +1319,6 @@ void AbstractShaderProgramGLTest::compute() {
 /* These are outside of any namespace to verify the macros fully qualify all
    names */
 namespace {
-    /* Yes, Clang, I know, the macros add unused members. That's fine. */
-    #ifdef CORRADE_TARGET_CLANG
-    #pragma GCC diagnostic push
-    #pragma GCC diagnostic ignored "-Wunused-member-function"
-    #endif
     struct ShaderSubclassDraw: Magnum::GL::AbstractShaderProgram {
         MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DRAW_IMPLEMENTATION(ShaderSubclassDraw)
     };
@@ -1332,9 +1327,6 @@ namespace {
         MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DISPATCH_IMPLEMENTATION(ShaderSubclassDispatch)
     };
     #endif
-    #ifdef CORRADE_TARGET_CLANG
-    #pragma GCC diagnostic pop
-    #endif
 }
 
 namespace Magnum { namespace GL { namespace Test { namespace {
@@ -1350,7 +1342,7 @@ void AbstractShaderProgramGLTest::subclassDraw() {
     Mesh meshNoInstances;
     meshNoInstances
         .setInstanceCount(0);
-    Mesh meshViewNoInstances;
+    MeshView meshViewNoInstances{mesh};
     meshViewNoInstances
         .setInstanceCount(0);
     #ifndef MAGNUM_TARGET_GLES