analysis: Add XML parsing support for external types

These are types either from C itself, or from another Vulkan library,
which are referenced in this XML file.

analysis/src/xml.rs

@@ -48,6 +48,7 @@ fn api_matches(node: &Node, expected: &str) -> bool {
 /// Raw representation of Vulkan XML files (`vk.xml`, `video.xml`).
 #[derive(Debug, Default)]
 pub struct Registry {
+    pub externals: Vec<External>,
     pub basetypes: Vec<BaseType>,
     pub bitmask_types: Vec<BitMaskType>,
     pub bitmask_aliases: Vec<Alias>,
@@ -127,6 +128,9 @@ impl Registry {
                                 Some("union") => {
                                     registry.unions.push(Structure::from_node(type_node, api));
                                 }
+                                None => {
+                                    registry.externals.push(External::from_node(type_node));
+                                }
                                 _ => (),
                             }
                         }
@@ -213,6 +217,21 @@ impl Alias {
     }
 }
 
+#[derive(Debug)]
+pub struct External {
+    pub name: XmlStr,
+    pub requires: Option<XmlStr>,
+}
+
+impl External {
+    fn from_node(node: Node) -> External {
+        External {
+            name: attribute(node, "name").unwrap(),
+            requires: attribute(node, "requires"),
+        }
+    }
+}
+
 #[derive(Debug)]
 pub struct BaseType {
     pub name: XmlStr,