reset clipping path when the state is restored

According to the specification, see NOTE 2 in
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf#G7.3882161

it appears that the clipping path should be reset
when the restore (Q) operator is encountered.

Fixes #739

poppler/Gfx.cc

diff --git a/poppler/Gfx.cc b/poppler/Gfx.cc
index f1c69b7..2db12d8 100644
--- a/poppler/Gfx.cc
+++ b/poppler/Gfx.cc
@@ -5380,6 +5380,7 @@
     state = state->restore();
     out->restoreState(state);
     stackHeight--;
+    clip = clipNone;
 }
 
 // Create a new state stack, and initialize it with a copy of the