Fix the fullscreen display rect for multiple monitors on Yosemite

[jquave]

When using a second display on Yosemite, the menu bar is not animated out as it is on the primary display, which causes the Mac OS menu bar to cover up the first line or so of MacVim in fullscreen. This fix defines a viewOffset method and adjusts the vim view rect based on whether or not the primary display is being used.

[chdiza]

This commit has caused two bugs.

The first is reported under here (Though this might not be caused by the commit in question; it's possible the commit just forgot to take account of non-Lion fullscreen mode).

The second is as follows. My gvimrc sets &columns and &lines to a specific dimension (80x40, respectively). I make an external monitor my primary one (it's active as soon as I plug it in to my MacBook), and the internal screen secondary. After this commit, if I click on the desktop of my secondary monitor to make it active, then Cmd-Tab to MacVim, then hit Cmd-N, I no longer get a 80x40 MacVim. Rather, I get a 68x25 MacVim. It's as if this commit makes MacVim assume that on a smaller monitor, the size of the MacVim window upon first creating one should be correspondingly smaller. If I then click over on the bigger desktop (for the external monitor) to make it active, then with MacVim the active app do Cmd-N, I get by 80x40 back.

So really, there are two sub-bugs to the second bug: (1) that on a smaller active screen, MacVim tries to make my window be smaller, and (2) that it somehow succeeds (how on earth is this overriding my gvimrc?).

[chdiza]

Ah, I have just discovered that the 68x25 nonsense is not the fault of this commit; I can reproduce that behavior in 7.4.622. My apologies.

(It is still a bug, though).