extmark: fix deletable nodes in MarkTree sometimes getting skipped

As per #14236, performing extmark cleanup in a certain namespace does
not guarantee removing all the extmarks inside given namespace.
The issue resides within the tree node removal method and results in
a couple of rare edge cases.

To demonstrate what causes this bug, I'll give an example covering one
of the edge cases.

=== AN EXAMPLE ===

   (A)         (B)         (C)         (D)         (E)
---------   ---------   ---------   ---------   ---------
  <0, 1>      <0, 1>      <0, 1>      <0, 1>      <0, 1>
  <0, 2>      <0, 2>      <0, 2>      <0, 2>      <0, 2>
  <0, 3>      <0, 3>      <0, 3>      <0, 3>      <0, 3>
  <0, 4>      <0, 4>      <0, 4>      <0, 4>      <0, 4>
  <0, 5>      <0, 5>      <0, 5>      <0, 5>      <0, 5>
  <0, 6>      <0, 6>      <0, 6>      <0, 6>      <0, 6>
  <0, 7>      <0, 7>      <0, 7>      <0, 7>      <0, 7>
  <0, 8>      <0, 8>      <0, 8>      <0, 8>      <0, 8>
  <0, 9>      <0, 9> *           *    <0, 9>  *   <0, 9>
[0, 10] *   [0, 10]     <0, 9>        [0, 11]     [0, 11]
  [0, 11]     [0, 11]     [0, 11]     [0, 12]     [0, 12]  *
  [0, 12]     [0, 12]     [0, 12]     [0, 13]     [0, 13]
  [0, 13]     [0, 13]     [0, 13]     [0, 14]     [0, 14]
  [0, 14]     [0, 14]     [0, 14]     [0, 15]     [0, 15]
  [0, 15]     [0, 15]     [0, 15]     [0, 16]     [0, 16]
  [0, 16]     [0, 16]     [0, 16]     [0, 17]     [0, 17]
  [0, 17]     [0, 17]     [0, 17]     [0, 18]     [0, 18]
  [0, 18]     [0, 18]     [0, 18]     [0, 19]     [0, 19]
  [0, 19]     [0, 19]     [0, 19]   [0, 20]     [0, 20]
[0, 20]     [0, 20]     [0, 20]

DIAGRAM EXPLANATION

* Every column is a state of the marktree at a certain stage.

* To make it simple, I don't draw the whole tree. What you see are
   2 leftmost parent nodes ([0, 10], [0, 20]) and their children placed
   in order `MarkTreeIter` would iterate through. From top to bottom.

* Numbers on this diagram represent extmark coordinates. Relative
   positioning and actual mark IDs used by the marktree are avoided
   for simplicity.

* 2 types of brackets around coordinates represent 2 different
   extmark namespaces (`ns_id`s).

* '*' shows iterator position.

ACTUAL EXPLANATION

Let's assume, we have two sets of extmarks from 2 different plugins:
  * Plugin1: <0, 1-9>
  * Plugin2: [0, 10-20]

1. Plugin2 calls
    `vim.api.nvim_buf_clear_namespace(buf_handle, ns_id, 0, -1)`
    to clear all its extmarks which results in `extmark_clear` call.

2. The iteration process goes on ignoring extmarks with irrelevant
    `ns_id` from Plugin1, until it reaches [0, 10], entering state (A).

3. At the end of cleaning up process, `marktree_del_itr` gets called.
    This function is supposed to remove given node and, if necessary,
    restructure the tree. Also, move the iterator to the next node.
    The bug occurs in this function.

4. The iterator goes backwards to the node's last child, to put it
    in the place of its deleted parent later. (B)

5. The parent node is deleted and replaced with its child node. (C)

6. Since now this node has 8 children, which is less than
    `MT_BRANCH_FACTOR - 1`, it get's merged with the next node. (D)

7. Finally, since at (B) the iterator went backward, it goes forward
    twice, skipping [0, 11] node, causing this extmark to persist,
    causing the bug. (E)

ANALYSIS AND SOLUTION

The algorithm works perfectly when the parent node gets replaced by
its child, but no merging occurs. I.e. the exact same diagram,
but without the (D) stage. If not for (D), it would iterate to <0, 9>
and then to [0, 11]. So, iterating twice makes sense. The actual problem
is in (C) stage, because the iterator index isn't adjusted and still
pointing to no longer existent node. So my solution is to adjust
iterator index after removing the child node.

More info: https://github.com/neovim/neovim/pull/14719

src/nvim/marktree.c

@@ -356,6 +356,7 @@ void marktree_del_itr(MarkTree *b, MarkTreeIter *itr, bool rev)
         y = y->level ? y->ptr[0] : NULL;
       }
     }
+    itr->i--;
   }
 
   b->n_keys--;

test/unit/marktree_spec.lua

@@ -186,5 +186,20 @@ describe('marktree', function()
       lib.marktree_check(tree)
       shadoworder(tree, shadow, iter2)
     end
+
+    -- Check iterator validity for 2 specific edge cases:
+    -- https://github.com/neovim/neovim/pull/14719
+    lib.marktree_clear(tree)
+    for i = 1,20 do
+      lib.marktree_put(tree, i, i, false)
+    end
+
+    lib.marktree_itr_get(tree, 10, 10, iter)
+    lib.marktree_del_itr(tree, iter, false)
+    eq(11, iter[0].node.key[iter[0].i].pos.col)
+
+    lib.marktree_itr_get(tree, 11, 11, iter)
+    lib.marktree_del_itr(tree, iter, false)
+    eq(12, iter[0].node.key[iter[0].i].pos.col)
  end)
 end)