macOS: window-step-resize should clamp initial window size to grid

[mitchellh]

Discussion: #2945

When window-step-resize is set, we set the macOS window increments but we don't set the original window size to be grid-clamped. After discussion, the expected behavior of a user of window-step-resize is that the window always remains clamped to the grid.

If window-step-resize is true, a new window should resize itself to perfectly fit the grid.

There are some odd behaviors with this: tabs and splits are allowed to have different font sizes. In Terminal.app, switching tabs between different font sizes actually changes the physical window size. This issue will remain focused on new windows only and ignore the tab/split problem for now. I will open other issues to track those behaviors.

(Related note: I think this is making me reconsider the config name window-step-resize because the behavior extends beyond simple resizing, but I'm not ready to paint that bike shed yet. This issue should focus on just resolving this one issue).