Add tests for PCLm

Files written in PCLm mode have to be created in a very specific way.
qpdf doesn't know how to create PCLm files from scratch. All it knows
how to do is to write an already valid file in a suitable way.
Therefore there is no command-line support for PCLm.

qpdf/qtest/qpdf.test

@@ -917,6 +917,19 @@ $td->runtest("check output",
 	     {$td->FILE => "from-scratch-0.pdf"});
 show_ntests();
 # ----------
+$td->notify("--- PCLm ---");
+$n_tests += 2;
+
+$td->runtest("write as PCLm",
+	     {$td->COMMAND => "test_driver 40 pclm-in.pdf a.pdf"},
+	     {$td->STRING => "test 40 done\n", $td->EXIT_STATUS => 0},
+	     $td->NORMALIZE_NEWLINES);
+$td->runtest("check output",
+	     {$td->FILE => "a.pdf"},
+	     {$td->FILE => "pclm-out.pdf"});
+
+show_ntests();
+# ----------
 $td->notify("--- Precheck streams ---");
 $n_tests += 2;
 

qpdf/test_driver.cc

@@ -1334,6 +1334,18 @@ void runtest(int n, char const* filename1, char const* arg2)
             }
         }
     }
+    else if (n == 40)
+    {
+        // Write PCLm. This requires specially crafted PDF files. This
+        // feature was implemented by Sahil Arora
+        // <sahilarora.535@gmail.com> as part of a Google Summer of
+        // Code project in 2017.
+        assert(arg2 != 0);
+        QPDFWriter w(pdf, arg2);
+        w.setPCLm(true);
+        w.setStaticID(true);
+        w.write();
+    }
     else
     {
 	throw std::runtime_error(std::string("invalid test ") +