Fix text position in drawSignatureFieldText

When centering vertically we calculate the y offset based on the height of the text and the annotation

When doing that we must ignore the border width, otherwise the text is offset downwards
diff --git a/poppler/Annot.cc b/poppler/Annot.cc
index 4e633fc..a33951a 100644
--- a/poppler/Annot.cc
+++ b/poppler/Annot.cc
@@ -5288,7 +5288,7 @@
     if (centerVertically) {
         const double outTextHeight = textCommands.nLines * da.getFontPtSize();
         if (outTextHeight < height) {
-            yDelta -= (height - outTextHeight) / 2;
+            yDelta = height - (height - outTextHeight) / 2;
         }
     }
     appendf("BT 1 0 0 1 {0:.2f} {1:.2f} Tm\n", leftMargin + textmargin, yDelta);