Shaders: fix assertion in MeshVisualizer.

It would break on GL 2.1 or when instantiating the shader on ES < 3.1
without wireframe enabled.

src/Magnum/Shaders/MeshVisualizer.cpp

@@ -60,10 +60,10 @@ MeshVisualizer::MeshVisualizer(const Flags flags): flags(flags), transformationP
 
     #ifndef MAGNUM_TARGET_GLES
     const Version version = Context::current()->supportedVersion({Version::GL320, Version::GL310, Version::GL300, Version::GL210});
-    CORRADE_INTERNAL_ASSERT(flags & Flag::NoGeometryShader || version >= Version::GL320);
+    CORRADE_INTERNAL_ASSERT(!flags || flags & Flag::NoGeometryShader || version >= Version::GL320);
     #else
     const Version version = Context::current()->supportedVersion({Version::GLES310, Version::GLES300, Version::GLES200});
-    CORRADE_INTERNAL_ASSERT(flags & Flag::NoGeometryShader || version >= Version::GLES310);
+    CORRADE_INTERNAL_ASSERT(!flags || flags & Flag::NoGeometryShader || version >= Version::GLES310);
     #endif
 
     Shader vert = Implementation::createCompatibilityShader(rs, version, Shader::Type::Vertex);