Move cursor to the top-left when returning to alternate screen

Fix broken preview border. Reported by Thomas Sattler.

    fzf --bind 'enter:execute(date)' --preview=date --reverse

src/tui/light.go

@@ -552,6 +552,9 @@ func (r *LightRenderer) Resume() bool {
 }
 
 func (r *LightRenderer) Clear() {
+  if r.fullscreen {
+    r.csi("H")
+  }
 	// r.csi("u")
 	r.origin()
 	r.csi("J")