[Mac] Add CocoaContext.GetGraphicsMode via CGL

Once the context is constructed, we use GetGraphicsMode to retrieve the
exact GraphicsMode that was selected by CocoaContext.
[Mac] Avoid class conflicts for multiple windows

When creating multiple windows, their classes must have different class
names or they will fail to construct.

This commit ensures their names are unique. It also fixes a potential
memory leak in the Icon setter.
Merge pull request #2 from thefiddler/cgl_cocoa_fixes

Cgl cocoa fixes