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/api/extmark.c

@@ -793,7 +793,7 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
   }
 
   // TODO(bfredl): synergize these two branches even more
-  if (ephemeral && decor_state.buf == buf) {
+  if (ephemeral && decor_state.win && decor_state.win->w_buffer == buf) {
     decor_add_ephemeral((int)line, (int)col, line2, col2, &decor, (uint64_t)ns_id, id);
   } else {
     if (ephemeral) {