[Platform] Make MouseCursor actor public
[Mac] Implement NSCursor rectangles
[Platform] Renamed MouseCursor.Rgba to Argb

This matches the default format of System.Drawing.Bitmap.
[OpenTK] Renamed x/y to hotx/hoty

The new name matches their functionality closer.
[OpenTK] Fixed compilation error
[Mac] Fixed cursor byte order
[Example] Test custom cursor
[SDL] Fixed custom MouseCursor
[SDL] Fixed MouseCursor getting stuck after multiple changes
[OpenTK] Rearranged MouseCursor parameters

MouseCursor and WindowIcon now match GL.TexImage2D in the way they
arrange their parameters. The expected values of each parameter are now
documented.
[SDL][Win] Destroy custom cursor on Close()

X11 destroys the cursor after setting it and Cocoa uses an autorelease
pool, so that should not be necessary there.
[Win] Only set MouseCursor in client area

This is required according to the windows documentation.
[Examples] Improved MouseCursor example
[Examples] Fixed cursor hotspot
[Examples] Use Key.Enter to capture mouse