do not store window size and position when in fullscreen mode

[Snaptags]

When using let g:screen_size_restore_pos = 1 in .vimrc to save and restore screen positions, gvim behaves strange when exiting from full screen mode: the "new" window dimensions are stored and gvim starts in some kind of full screen view the next time. This is ugly, because the vim-fullscreen full screen mode does look much better and can be set as default via .vimrc.

I made several small modification to:

• disable gvim's screen size saving when full screen is activated
• restore the original setting of screen_size_restore_pos when full screen is deactivated

I successfully tested this using Windows only, but should not break anything on other environments.