refactor(sign): store 'signcolumn' width range when it is set

Problem:  Minimum and maximum signcolumn width is determined each redraw.
Solution: Determine and store 'signcolumn' range when option is set.

src/nvim/buffer_defs.h

@@ -1206,6 +1206,8 @@ struct window_S {
   int w_nrwidth;                    // width of 'number' and 'relativenumber'
                                     // column being used
   int w_scwidth;                    // width of 'signcolumn'
+  int w_minscwidth;                 // minimum width or SCL_NO/SCL_NUM
+  int w_maxscwidth;                 // maximum width or SCL_NO/SCL_NUM
 
   // === end of cached values ===