Split `NK_WINDOW_NO_SCROLLBAR` into 2 flags

[PROP65]

Splits NK_WINDOW_NO_SCROLLBAR into separate horizontal and vertical flags.

/* Removes the horizontal scrollbar from the window */
NK_WINDOW_NO_SCROLLBAR_H = NK_FLAG(5)
/* Removes the vertical scrollbar from the window */
NK_WINDOW_NO_SCROLLBAR_V = NK_FLAG(6)
/* Removes both scrollbars from the window */
NK_WINDOW_NO_SCROLLBAR   = NK_WINDOW_NO_SCROLLBAR_H | NK_WINDOW_NO_SCROLLBAR_V

Needs more testing.