drm/radeon: Fix build of radeon_fb.c on i386
o Simplify POS_INDEX macro calculation.
o New macro POS_COPY to copy between term_pos_t.
o Add vtbuf_wth/vtbuf_htw helpers, to translate between screen coordinates and
	circular history buffer location.
o Update vtbuf_iscursor to mark region selected by mouse.
o New helper vtbuf_flush_mark, to update regions where copy/paste mark changed.
o New method vtbuf_get_marked_len to get storage size for paste buffer.
o vtbuf_extract_marked fill (caller allocated buffer) with selected region data.
o Simplify mouse handler for copy/paste, and use vtbuf_flush_mark to update.
o New method vtbuf_scroll_mode, to help indicate Scroll mode by hiding cursor.
o Update header with new vtbuf methods.
o Add new vt_driver method vd_markedwin, to hold last window with selection.
o Enable paste support in core module.

Sponsored by:	The FreeBSD Foundation
Pointed by:	Claude Buisson <clbuisson@orange.fr> (Scroll mode indication)
Move vt_mouse_event out of sysmouse lock.

Sponsored by:	The FreeBSD Foundation