Added FPDFAnnot_GetAnnotIndex()

Added FPDFAnnot_GetAnnotIndex() to get the index of an annotation. This
is useful if linked annotations are renedered together - then we need to
know which ones in the annotation list we need to skip.

Bug=pdfium:863,pdfium:737

Change-Id: I53482a15e0fd9a896b348b64d68e99f9c21da9f9
Reviewed-on: https://pdfium-review.googlesource.com/11970
Commit-Queue: dsinclair <dsinclair@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
diff --git a/public/fpdf_annot.h b/public/fpdf_annot.h
index 6a89ff9..429a3f7 100644
--- a/public/fpdf_annot.h
+++ b/public/fpdf_annot.h
@@ -134,6 +134,17 @@
                                                             int index);
 
 // Experimental API.
+// Get the index of |annot| in |page|. This is the opposite of
+// FPDFPage_GetAnnot().
+//
+//   page  - handle to the page that the annotation is on.
+//   annot - handle to an annotation.
+//
+// Returns the index of |annot|, or -1 on failure.
+FPDF_EXPORT int FPDF_CALLCONV FPDFPage_GetAnnotIndex(FPDF_PAGE page,
+                                                     FPDF_ANNOTATION annot);
+
+// Experimental API.
 // Close an annotation. Must be called when the annotation returned by
 // FPDFPage_CreateAnnot() or FPDFPage_GetAnnot() is no longer needed. This
 // function does not remove the annotation from the document.