Updated OpenTK.dll to 1.1.1445.5363

Upstream repository has moved to https://github.com/opentk/opentk. This OpenTK.dll is based on
https://github.com/opentk/opentk/commit/3ee6d1dbbdbb1e06f41e18e9cc86b2febeb57bd8

Significant changes:
- support for OpenGL 4.4
- support for OpenGL ES 3.0
- SDL2 backend
- significantly faster startup times (3x-10x depending on configuration)
- faster interop with generic methods (e.g. GL.[Get]BufferData,
GL.[Get]TexImage)
- lower memory overhead (670KB vs 2800KB)
- support for high-DPI modes
- stability improvements on Windows, Linux and Mac OS X

The SDL2 backend is used automatically when SDL2 is detected. If SDL2 is not available, the native backends will be used (WGL, GLX, AGL). The fallback behavior can be configured at runtime.