refactor(extmarks): some minor internal API changes

extranges and a bunch of other improvements are coming for 0.10
This gets in some minor surrounding API changes to avoid rebase
conflicts until then.

- decorations will be able to be specific to windows
- adjust deletion API to fit with extranges

src/nvim/marktree.h

@@ -22,21 +22,26 @@ typedef struct {
   int32_t row;
   int32_t col;
 } mtpos_t;
+#define mtpos_t(r, c) ((mtpos_t){ .row = (r), .col = (c) })
 
 typedef struct mtnode_s mtnode_t;
-typedef struct {
-  int oldcol;
-  int i;
-} iterstate_t;
 
 typedef struct {
   mtpos_t pos;
   int lvl;
   mtnode_t *node;
   int i;
-  iterstate_t s[MT_MAX_DEPTH];
+  struct {
+    int oldcol;
+    int i;
+  } s[MT_MAX_DEPTH];
+
+  size_t intersect_idx;
+  mtpos_t intersect_pos;
 } MarkTreeIter;
 
+#define marktree_itr_valid(itr) ((itr)->node != NULL)
+
 // Internal storage
 //
 // NB: actual marks have flags > 0, so we can use (row,col,0) pseudo-key for
@@ -123,8 +128,6 @@ struct mtnode_s {
   mtnode_t *ptr[];
 };
 
-// TODO(bfredl): the iterator is pretty much everpresent, make it part of the
-// tree struct itself?
 typedef struct {
   mtnode_t *root;
   size_t n_keys, n_nodes;