Reflowing the grid in-place involved way too much memmove() for a big

performance cost with a large history. Instead change back to using a
second grid and copying modified lines over which is much faster (this
doesn't revert to the old code however which didn't support UTF-8
properly). GitHub issue 1249.

tmux.h

@@ -551,6 +551,7 @@ enum utf8_state {
 /* Grid line flags. */
 #define GRID_LINE_WRAPPED 0x1
 #define GRID_LINE_EXTENDED 0x2
+#define GRID_LINE_DEAD 0x4
 
 /* Grid cell data. */
 struct grid_cell {