LibPDF: Use a RAII object to restore state in recursive render

Previously, if one operator returned an error, the TRY() would cause
us to return without restoring the outer graphics state, leading to
problems such as handing a 3-tuple to a grayscale color space
(because the inner object set up a grayscale color space that we
failed to dispose of).

Makes us crash later on page 43 of
https://devstreaming-cdn.apple.com/videos/wwdc/2017/821kjtggolzxsv/821/821_get_started_with_display_p3.pdf

Userland/Libraries/LibPDF/Renderer.cpp

@@ -644,7 +644,25 @@ RENDERER_HANDLER(paint_xobject)
         return {};
     }
 
-    MUST(handle_save_state({}));
+    // Use a RAII object to restore the graphics state, to make sure it gets restored even if
+    // a TRY(handle_operator()) causes us to exit the operators loop early.
+    class ScopedState {
+    public:
+        ScopedState(Renderer& renderer)
+            : m_renderer(renderer)
+        {
+            MUST(m_renderer.handle_save_state({}));
+        }
+        ~ScopedState()
+        {
+            MUST(m_renderer.handle_restore_state({}));
+        }
+
+    private:
+        Renderer& m_renderer;
+    };
+    ScopedState scoped_state { *this };
+
     Vector<Value> matrix;
     if (xobject->dict()->contains(CommonNames::Matrix)) {
         matrix = xobject->dict()->get_array(m_document, CommonNames::Matrix).value()->elements();
@@ -655,7 +673,6 @@ RENDERER_HANDLER(paint_xobject)
     auto operators = TRY(Parser::parse_operators(m_document, xobject->bytes()));
     for (auto& op : operators)
         TRY(handle_operator(op, xobject_resources));
-    MUST(handle_restore_state({}));
     return {};
 }