perf(column): keep track of number of lines that hold up the 'signcolumn'

Problem:  The entire marktree needs to be traversed each time a sign is
          removed from the sentinel line.
Solution: Remove sentinel line and instead keep track of the number of
          lines that hold up the 'signcolumn' in "max_count". Adjust this
          number for added/removed signs, and set it to 0 when the
          maximum number of signs on a line changes. Only when
          "max_count" is decremented to 0 due to sign removal do we need
          to check the entire buffer.

          Also replace "invalid_top" and "invalid_bot" with a map of
          invalid ranges, further reducing the number of lines to be
          checked.

          Also improve tree traversal when counting the number of signs.
          Instead of looping over the to be checked range and counting
          the overlap for each row, keep track of the overlap in an
          array and add this to the count.

src/nvim/buffer_defs.h

@@ -703,11 +703,10 @@ struct file_buffer {
                                 // may use a different synblock_T.
 
   struct {
-    int size;                   // last calculated number of sign columns
-    int max;                    // maximum value size is valid for.
-    linenr_T sentinel;          // a line number which is holding up the signcolumn
-    linenr_T invalid_top;       // first invalid line number that needs to be checked
-    linenr_T invalid_bot;       // last invalid line number that needs to be checked
+    int max;                         // maximum number of signs on a single line
+    int max_count;                   // number of lines with max number of signs
+    bool resized;                    // whether max changed at start of redraw
+    Map(int, SignRange) invalid[1];  // map of invalid ranges to be checked
   } b_signcols;
 
   Terminal *terminal;           // Terminal instance associated with the buffer