InputSource::unreadCh -- only unread most recently read character

This is all that ever worked. The test suite was trying to do
something different from ClosedFileInputSource.

TODO

@@ -315,6 +315,12 @@ I find it useful to make reference to them in this list
  * Investigate whether there is a way to automate the memory checker
    tests for Windows.
 
+ * Part of closed_file_input_source.cc is disabled on Windows because
+   of odd failures. It might be worth investigating so we can fully
+   exercise this in the test suite. That said, ClosedFileInputSource
+   is exercised elsewhere in qpdf's test suite, so this is not that
+   pressing.
+
  * Support user-pluggable stream filters.  This would enable external
    code to provide interpretation for filters that are missing from
    qpdf.  Make it possible for user-provided filters to override

include/qpdf/InputSource.hh

@@ -83,6 +83,9 @@ class QPDF_DLL_CLASS InputSource
     virtual void seek(qpdf_offset_t offset, int whence) = 0;
     virtual void rewind() = 0;
     virtual size_t read(char* buffer, size_t length) = 0;
+
+    // Note: you can only unread the character you just read. The
+    // specific character is ignored by some implementations.
     virtual void unreadCh(char ch) = 0;
 
   protected:

libtests/closed_file_input_source.cc

@@ -38,9 +38,9 @@ void do_tests(InputSource* is)
     is->seek(521, SEEK_SET);
     is->read(b, 1);
 #else
-    is->unreadCh('Q');
+    is->unreadCh('\n');
     check("read unread character", 1 == is->read(b, 1));
-    check("got character", 'Q' == b[0]);
+    check("got character", '\n' == b[0]);
 #endif
     check("last offset after read unread", 521 == is->getLastOffset());
     is->seek(0, SEEK_END);