Update OpenTK to 1.1.2

This release contains important fixes for MonoGame:
- New Cocoa backend for Mac OS X
- New Mouse.GetCursorState() API that matches XNA Mouse.GetState()
- New GameWindow.Cursor API that matches XNA behavior
- Improved Linux/X11 input API:
  - Mouse.SetPosition now works correctly
  - non-US keyboard layouts now work correctly
- Improved stability when using the EGL/ANGLE backend
- Improved stability when using the SDL2 backend
- Improved stability when running on a headless system
- Up to 20x better startup performance (5-10ms compared to 150ms)
- Up to 10x smaller allocation graph (~900 objects compared to ~9000)

This PR is a prerequisite for https://github.com/mono/MonoGame/issues/2464, https://github.com/mono/MonoGame/issues/2422 and a number of other issues.