shader: fix shader newline error with Intel drivers

It appears that Intel drivers always need a newline at the end of the file.

panda/src/gobj/shader.cxx

@@ -2497,6 +2497,9 @@ do_read_source(string &into, const Filename &fn, BamCacheRecord *record) {
     into.resize(into.size() - 1);
   }
 
+  // Except add back a newline at the end, which is needed by Intel drivers.
+  into += "\n";
+
   return true;
 }