Do not call monotonic() when changing cursor position

monotonic() is extremely slow. This call was halving the CSI parsing
speed benchmark. Instead we use the time at which parsing of the current
input chunk was started. This should be within a few microseconds and
accurate enough for the cursor trail for which it is used.

kitty/screen.c

@@ -1805,7 +1805,7 @@ screen_cursor_position(Screen *self, unsigned int line, unsigned int column) {
         line += self->margin_top;
         line = MAX(self->margin_top, MIN(line, self->margin_bottom));
     }
-    self->cursor->position_changed_by_client_at = monotonic();
+    self->cursor->position_changed_by_client_at = self->parsing_at;
     self->cursor->x = column; self->cursor->y = line;
     screen_ensure_bounds(self, false, in_margins);
 }

kitty/screen.h

@@ -173,6 +173,7 @@ typedef struct {
     } paused_rendering;
     CharsetState charset;
     ListOfChars *lc;
+    monotonic_t parsing_at;
 } Screen;
 
 

kitty/vt-parser.c

@@ -1417,6 +1417,7 @@ static void
 run_worker(void *p, ParseData *pd, bool flush) {
     Screen *screen = (Screen*)p;
     PS *self = (PS*)screen->vt_parser->state;
+    screen->parsing_at = pd->now;
     with_lock {
         self->read.sz += self->write.pending; self->write.pending = 0;
         pd->has_pending_input = self->read.pos < self->read.sz;