refactor(signcol): smarter invalidation (#17533)

Previously b_signcols was invalidated whenever a sign was added/removed
or when a buffer line was added/removed.

This change introduces a sentinel linenr_T into the buffer state which
is a line number used to determine the signcolumn. With this
information, we can invalidate the signcolumn less often. Now the
signcolumn is only invalidated when a sign or line at the sentinel line
number is removed.

src/nvim/buffer_defs.h

@@ -862,9 +862,12 @@ struct file_buffer {
                                 // may use a different synblock_T.
 
   sign_entry_T *b_signlist;     // list of placed signs
-  int b_signcols;               // last calculated number of sign columns
-  bool b_signcols_valid;        // calculated sign columns is valid
-  int b_signcols_max;           // Maximum value b_signcols is valid for.
+  struct {
+    int size;                   // last calculated number of sign columns
+    bool valid;                 // calculated sign columns is valid
+    linenr_T sentinel;          // a line number which is holding up the signcolumn
+    int max;                    // Maximum value size is valid for.
+  } b_signcols;
 
   Terminal *terminal;           // Terminal instance associated with the buffer