Refactor PDF canvas transform to use Save/Restore state

Move Y-axis flip transform inside signature drawing block and
wrap with SaveState/RestoreState to prevent side effects on
other drawing operations and pages. Removes redundant
page-level transformation for better encapsulation.

EnvelopeGenerator.PdfEditor/Pdf.cs

@@ -105,17 +105,13 @@ namespace EnvelopeGenerator.PdfEditor
         public Pdf<TInputStream, TOutputStream> Background<TSignature>(IEnumerable<TSignature> signatures, double widthPx = 1.9500000000000002, double heightPx = 2.52)
             where TSignature : ISignature
         {
-            // once per page
-            Page(page =>
-            {
-                var canvas = new PdfCanvas(page);
-                canvas.ConcatMatrix(1, 0, 0, -1, 0, page.GetPageSize().GetHeight());
-            });
-
             foreach (var signature in signatures)
                 Page(signature.Page, page =>
                 {
                     var canvas = new PdfCanvas(page);
+                    canvas.SaveState();
+                    canvas.ConcatMatrix(1, 0, 0, -1, 0, page.GetPageSize().GetHeight());
+
                     double inchFactor = 72;
                     double magin = .2;
                     double x = (signature.X - .7 - magin) * inchFactor;
@@ -134,6 +130,8 @@ namespace EnvelopeGenerator.PdfEditor
                     canvas.SetFillColor(new DeviceRgb(204, 202, 198))
                         .Rectangle(x, y + height - bottomLineLength, width, bottomLineLength)
                         .Fill();
+
+                    canvas.RestoreState();
                 });
 
             return this;