Initialize all bitfields to zero in generated constructors

This currently requires bitfields to be defined with the <bitfield> tag -
currently, all bitfield fields with a type-name use <compound> instead.

DFHack/dfhack#1047 (partial fix)
DFHack/dfhack#1038
DFHack#169
DFHack#171

StructFields.pm

@@ -348,8 +348,8 @@ sub render_field_init($) {
     } elsif ($meta eq 'global' || $meta eq 'compound') {
         return unless $subtype;
 
-        if ($subtype eq 'bitfield' && defined $cur_init_value) {
-            emit $cur_init_field, '.whole = ', $cur_init_value, ';';
+        if ($subtype eq 'bitfield') {
+            emit $cur_init_field, '.whole = ', ($cur_init_value || 0), ';';
         } elsif ($subtype eq 'enum') {
             if ($meta eq 'global') {
                 my $tname = $field->getAttribute('type-name');