Text: test that calling Renderer2D::render() replaces the contents.

Apart from the fact that it blew up when the text was indeed empty,
which is fixed now (ugh), it was kinda obvious with the current
implementation but won't be when the guts get replaced with something
reasonable. So ensure we don't break existing use cases.

src/Magnum/Text/Renderer.cpp

@@ -2044,12 +2044,14 @@ void AbstractRenderer::render(const std::string& text) {
     CORRADE_ASSERT(glyphCount <= _capacity,
         "Text::Renderer::render(): capacity" << _capacity << "too small to render" << glyphCount << "glyphs", );
 
-    /* Interleave the data into mapped buffer*/
-    Containers::ArrayView<Vertex> vertices(static_cast<Vertex*>(bufferMapImplementation(_vertexBuffer,
-        vertexCount*sizeof(Vertex))), vertexCount);
-    CORRADE_INTERNAL_ASSERT_OUTPUT(vertices);
-    std::copy(vertexData.begin(), vertexData.end(), vertices.begin());
-    bufferUnmapImplementation(_vertexBuffer);
+    /* Interleave the data into mapped buffer, if there are any glyphs */
+    if(vertexCount) {
+        Containers::ArrayView<Vertex> vertices(static_cast<Vertex*>(bufferMapImplementation(_vertexBuffer,
+            vertexCount*sizeof(Vertex))), vertexCount);
+        CORRADE_INTERNAL_ASSERT_OUTPUT(vertices);
+        std::copy(vertexData.begin(), vertexData.end(), vertices.begin());
+        bufferUnmapImplementation(_vertexBuffer);
+    }
 
     /* Update index count */
     _mesh.setCount(indexCount);

src/Magnum/Text/Test/RendererGLTest.cpp

@@ -1147,6 +1147,12 @@ void RendererGLTest::mutableText() {
         Vector2{ 9.0f,  9.0f} + offset, {1.0f, 1.0f},
     }), TestSuite::Compare::Container);
     #endif
+
+    /* Rendering again replaces previous contents. I.e., if rendering an empty
+       string, it makes the output empty. */
+    renderer.render("");
+    CORRADE_COMPARE(renderer.rectangle(), (Range2D{{0.0f, -1.75f}, {0.0f, 1.75f}}));
+    CORRADE_COMPARE(renderer.mesh().count(), 0);
 }
 
 void RendererGLTest::arrayGlyphCache() {