Previously, we were using the window's top-left position to determine
the scale to use for the OpenGLContext. However, on macOS the
backingScaleFactor of the window is not strictly related to the top-left
corner of the window, so the OpenGL view's scale could end up differing
from the backing scale factor when slowly moving a window between
displays with different backing scale factors.

On macOS, we now use the backing scale factor of the window's screen (as
maintained by AppKit), rather than trying to work out the correct
display and scale ourselves.

When the AvailableModulesList is destroyed, it will wait for the current
module scanning job (running on a background thread) to complete. This
job will try to take the AvailableModulesList::lock when it completes,
so the lock must outlive the background thread.