OpenTK 3.1.0

General:
* Fixed problem where OpenGL 3.2 contexts where created using the wrong flags, causing renderdoc error (#790).
* Fixed problem where Vector3.Transform(Quaternion) returned incorrect results ( #776 ).
* SDL backend no longer handles exceptions thrown in input events ( #735 #737 )
* Updated gamepadd mappings for SDL (#927).
* Alt + numpad now results in correct data being passed to OnKeyPress (f17fa4b).
Windows:
* Keypress events are using CharSet.Unicode to allow for UTF characters.
* If GetMouseMovePointsEx returns "access denied" we fall back to the old mouse move handling ( #883 ).
* Detect joysticks which declare no valid controls & ignore (Cherry-picked from #819 ).
* Fix where an invalid joystick axis overwrites the first joystick axis. (Cherry-picked from #819 ).
* Makes the ArbCreateContext return valid settings ( #754 ).
MacOS:
* Unprocessed events no longer put the application into a partially-activated state ( #732 ).
* Multiple fixes under PR #914:
* NativeWindow.CursorVisible no longer resets the mouse position ( #668 ).
* Custom cursors should work now ( e598ab2 ).
* Command key is now a valid modifier ( 286119e ).
* Fix where KeyDown events would trigger KeyPress when they shouldn't, e.g. when pasting ( ea3dd48 ).
* KeyPress now receives the correct keycodes even if modifiers where held when typing ( 91b03dd ).
Internal:
* Generator.Build now uses invariant culture for parsing avoiding an exception when the local system uses comma as decimal separator ( #750 ).
* Removed weird assembly version check fixing #710.
* Fixed Xamarin project so that it compiles ( See #725 and b16e7fa ).
* Matrix4 uses unsafe to invert for performace ( #719 ).
* Removed link to gitter chat, discord is where it's at ( #770 ).