Fix #118694: snap cursor sometimes doesn't update when dragging and dropping assets

The problem happens because, on some OSs, `wm->winactive->eventstate`
does not update when the mouse moves. So the snap cursor doesn't update
because apparently the mouse didn't move.

The solution in this commit is to use the `eventstate` of the window in
the context (`CTX_wm_window(C)->eventstate`).

This matches other code snippets where `eventstate` is used.

---
The behavior on each OS apparently happens like this:
**Windows:**
- `wm->winactive` is updated if the cursor is over the window for a little less than 1 second
- Events are sent to the window whose cursor is over.

**MacOS:**
- `wm->winactive` is the last one selected
- Events are sent to the active window and to the one whose cursor is over.

**Linux:**
- `wm->winactive` is the last one selected
- Events are sent to the window whose cursor is over.

Therefore, we cannot trust the eventstate of `wm->winactive`.

@@ -490,3 +490,3 @@
 static bool v3d_cursor_eventstate_has_changed(SnapCursorDataIntern *data_intern,
                                               V3DSnapCursorState *state,
-                                              const wmWindowManager *wm,
+                                              const wmEvent *event,

@@ -606,3 +608,3 @@
 static void v3d_cursor_snap_update(V3DSnapCursorState *state,
                                    const bContext *C,
-                                   wmWindowManager *wm,
+                                   const wmEvent *event,

@@ -867,2 +857,4 @@
+  wmWindow *win = CTX_wm_window(C);
   ScrArea *area = CTX_wm_area(C);
   ARegion *region = BKE_area_find_region_type(area, RGN_TYPE_WINDOW);
+  uint8_t event_modifier = win->eventstate ? win->eventstate->modifier : KM_NOTHING;