The OpenGL renderer uses ColourGradient::createLookupTable to generate
gradient textures. However, the tweening method used was different to
the tweening used by CoreGraphics gradients, and by the software
renderer.

Gradient tweening is now computed using non-premultiplied colours, to
ensure consistency between gradients rendered using OpenGL, and with
other renderers.

- 4.1 and 4.3 contexts can now be requested
- The requested context version is no longer ignored on Linux
- Debugging contexts are now enabled in Debug builds with GL 4.3
- Fixes a bug where glEnable(GL_TEXTURE_2D) was called in core profiles

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 */ });

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