Previously, it wasn't safe to access Font instances from multiple
threads because there was a chance that they might reference the same
shared internal state. In this case, calling getTypeface() or getAscent from
two threads simultaneously would cause a race on the typeface and ascent
data members, even though the Font instances appeared to be disjoint.

With this change in place, it is now safe to use Font instances from
multiple threads simultaneously.

It is still an error to modify the same Font instance from multiple
threads without synchronization!

    // Fine:
    Font a;
    Font b = a;

    auto futureA = std::async (std::launch::async, [&a] { /* do something with a */ });
    auto futureB = std::async (std::launch::async, [&b] { /* do something with b */ });

    // Bad idea:
    Font f;

    auto futureA = std::async (std::launch::async, [&f] { /* do something with f */ });
    auto futureB = std::async (std::launch::async, [&f] { /* do something with f */ });

In particularly unfortunate cases, the fonts used for laying out and
drawing an AttributedString could end up differing, meaning that
incorrect glyph indices would be used during rendering. The rendered
text would end up using incorrect character symbols, making it
unreadable.

The problem seems to be:
- The AttributedString fetched a typeface during the getOrCreateFont
  call in createCFAttributedString.
- Details of the typeface (its style and name) were stored into the
  CFAttributedString.
- During `createLayout`, a new `Font` object was created using the
  stored typeface name and style.
- When calling `getTypeface` on this new font, during rendering,
  a different typeface was returned.

The fix implemented here stores the Font objects used during the
creating of the CFAttributedString, so that the fonts written to
the juce::AttributedString are guaranteed to match the fonts used
to compute the layout.

You can override this by defining JUCE_FORCE_USE_NATIVE_TEXT_LAYOUT_FOR_MEMORY_FONTS=1

You can override this by defining JUCE_FORCE_USE_NATIVE_TEXT_LAYOUT_FOR_MEMORY_FONTS=1