Add cursor mode GLFW_CURSOR_CAPTURED

This adds a cursor mode that provides a visible cursor confined to the
content area of the window.

Fixes #58

src/wl_platform.h

@@ -269,6 +269,7 @@ typedef struct _GLFWwindowWayland
 
     struct zwp_relative_pointer_v1* relativePointer;
     struct zwp_locked_pointer_v1*   lockedPointer;
+    struct zwp_confined_pointer_v1* confinedPointer;
 
     struct zwp_idle_inhibitor_v1*          idleInhibitor;