ui: Fix redraw bug caused by race conditions with remote clients

Before sending a resize command to the UIs, flush the current output buffer to
ensure no redraw commands for a screen with invalid size are processed.

src/nvim/screen.c

@@ -8188,6 +8188,9 @@ void screen_resize(int width, int height, int mustset)
   check_shellsize();
 
   if (abstract_ui) {
+    // Clear the output buffer to ensure UIs don't receive redraw command meant
+    // for invalid screen sizes.
+    out_buf_clear();
     ui_resize(width, height);
   } else {
     mch_set_shellsize();

src/nvim/term.c

@@ -1817,6 +1817,12 @@ void term_write(char_u *s, size_t len)
 static char_u out_buf[OUT_SIZE + 1];
 static int out_pos = 0;                 /* number of chars in out_buf */
 
+// Clear the output buffer
+void out_buf_clear(void)
+{
+  out_pos = 0;
+}
+
 /*
  * out_flush(): flush the output buffer
  */

src/nvim/ui.c

@@ -173,7 +173,7 @@ void ui_refresh(void)
     return;
   }
 
-  width = height = INT_MAX;
+  int width = INT_MAX, height = INT_MAX;
 
   for (size_t i = 0; i < ui_count; i++) {
     UI *ui = uis[i];