Added some assertions to catch use of some Font methods on background threads

modules/juce_graphics/fonts/juce_Font.cpp

@@ -625,6 +625,10 @@ int Font::getStringWidth (const String& text) const
 
 float Font::getStringWidthFloat (const String& text) const
 {
+    // This call isn't thread-safe when there's a message thread running
+    jassert (MessageManager::getInstanceWithoutCreating() == nullptr
+               || MessageManager::getInstanceWithoutCreating()->currentThreadHasLockedMessageManager());
+
     auto w = getTypeface()->getStringWidth (text);
 
     if (font->kerning != 0.0f)
@@ -635,6 +639,10 @@ float Font::getStringWidthFloat (const String& text) const
 
 void Font::getGlyphPositions (const String& text, Array<int>& glyphs, Array<float>& xOffsets) const
 {
+    // This call isn't thread-safe when there's a message thread running
+    jassert (MessageManager::getInstanceWithoutCreating() == nullptr
+               || MessageManager::getInstanceWithoutCreating()->currentThreadHasLockedMessageManager());
+
     getTypeface()->getGlyphPositions (text, glyphs, xOffsets);
 
     if (auto num = xOffsets.size())