Previously, Xcode's main thread checker would complain when selecting
the "OpenGL Renderer" in the DemoRunner on macOS.

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 */ });

See the breaking changes document for more details.

Previously, we were always adding a #version 150 directive to shaders
when the GLSL version was higher than 1.2, which could cause compilation
to fail on platforms with GLSL versions between 1.2 and 1.5.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.