Add new public APIs to find the z-order for links and widgets.

core/src/fpdfdoc/doc_form.cpp

Index: core/src/fpdfdoc/doc_form.cpp
diff --git a/core/src/fpdfdoc/doc_form.cpp b/core/src/fpdfdoc/doc_form.cpp
index a59f35eeac5139429d05897221f7078254981801..64e9b039f90e2cf03a68094ab929580626df9fa9 100644
--- a/core/src/fpdfdoc/doc_form.cpp
+++ b/core/src/fpdfdoc/doc_form.cpp
@@ -866,29 +866,36 @@ CPDF_FormControl* CPDF_InterForm::GetPageControl(CPDF_Page* pPage,
   }
   return NULL;
 }
+
 CPDF_FormControl* CPDF_InterForm::GetControlAtPoint(CPDF_Page* pPage,
                                                     FX_FLOAT pdf_x,
-                                                    FX_FLOAT pdf_y) const {
+                                                    FX_FLOAT pdf_y,
+                                                    int* z_order) const {
   CPDF_Array* pAnnotList = pPage->m_pFormDict->GetArray("Annots");
-  if (pAnnotList == NULL) {
-    return NULL;
-  }
-  for (FX_DWORD i = pAnnotList->GetCount(); i > 0; i--) {
-    CPDF_Dictionary* pAnnot = pAnnotList->GetDict(i - 1);
-    if (pAnnot == NULL) {
+  if (!pAnnotList)
+    return nullptr;
+
+  for (FX_DWORD i = pAnnotList->GetCount(); i > 0; --i) {
+    FX_DWORD annot_index = i - 1;
+    CPDF_Dictionary* pAnnot = pAnnotList->GetDict(annot_index);
+    if (!pAnnot)
       continue;
-    }
+
     CPDF_FormControl* pControl;
-    if (!m_ControlMap.Lookup(pAnnot, (void*&)pControl)) {
+    if (!m_ControlMap.Lookup(pAnnot, (void*&)pControl))
       continue;
-    }
+
     CFX_FloatRect rect = pControl->GetRect();
-    if (rect.Contains(pdf_x, pdf_y)) {
-      return pControl;
-    }
+    if (!rect.Contains(pdf_x, pdf_y))
+      continue;
+
+    if (z_order)
+      *z_order = annot_index;
+    return pControl;
   }
-  return NULL;
+  return nullptr;
 }
+
 CPDF_FormControl* CPDF_InterForm::GetControlByDict(
     CPDF_Dictionary* pWidgetDict) const {
   CPDF_FormControl* pControl = NULL;