Add GLFW_CENTER_CURSOR

Adds a hint for controlling whether the cursor is centered over newly
created full screen windows.

Fixes #749.
Closes #842.

include/GLFW/glfw3.h

@@ -686,6 +686,7 @@ extern "C" {
  *  [window attribute](@ref GLFW_MAXIMIZED_attrib).
  */
 #define GLFW_MAXIMIZED              0x00020008
+#define GLFW_CENTER_CURSOR          0x00020009
 
 /*! @brief Framebuffer bit depth hint.
  *