Previously, if the PathFlatteningIterator returned a line segment with
very large x or y positions, the result of
    roundToInt (iter.y1 * 256.0f)
could be incorrect, if the result was too large to fit in an int.

Using int64_t to store intermediate results, converting to int when
updating the edge table itself produces accurate results for a wider
range of inputs.

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.

CGContextGetClipBoundingBox sometimes returns a 'null' rect. When it
does, the following logic will get stuck in the while loop drawing
individual tiles over and over again.

Alternatively we could disable antialiasing before the fill operation
but this could cause neighbouring Components to overlap on the screen
even if their coordinates don't.

The current minimum-supported Xcode (10.1) includes the macOS 10.14.1
SDK, so APIs from macOS 10.14 will always be available.

This also fixes an issue where the requested font style could be
incorrectly overwritten.