cocoa_common: avoid window creation with VOFLAG_HIDDEN

This takes an approach similar to the wayland OpenGL backend. VOFLAG_HIDDEN
flag semantics doesn't mean "hide the window" but is simply ever used only to
do detection of available OpenGL extensions. On OSX it's possibile to
accomplish this task just by creating the OpenGL context without attaching
it to a drawable.
cocoa_common: don't autohide mouse on mousedown

Previews code allowed to click the same spot for a long time and the cursor
would autohide. No more!