Changed xgbgen to support xcb-proto 1.12

* Added minimal support for switch fields.
* Changed the way Size is calculated to accomodate for lists inside structs
  (added to randr)
* Removed heuristic to place alignment gaps, they are now explicitly
  described in xml

nexgb/xgbgen/xml_fields.go

@@ -10,6 +10,7 @@ type XMLField struct {
 
 	// For 'pad' element
 	Bytes uint `xml:"bytes,attr"`
+	Align uint16 `xml:"align,attr"`
 
 	// For 'field', 'list', 'localfield', 'exprfield' and 'switch' elements.
 	Name string `xml:"name,attr"`