Added set_cursor_position and hide_cursor functions

[gufoe]

Hello, I am trying to develop a simple 3d first person shooter game, and I need these two functions.

The problem:
I cannot use the set_cursor_grab on its own, because if the mouse is already on a edge of the window (e.g. 0-300), and I move the mouse left, the WindowEvent::CursorPos will still return 0-300 so i canont detect the fact that the user is trying to move the mouse left (am I wrong?).

Using set_cursor_position in conjunction with set_cursor_grab solves the problem as I'm now able to center the mouse.

[gufoe]

Note that the build failed for external reasons:
Could not compile matrixmultiply.

same on #197

[sebcrozet]

Thanks!