attribution: in-game ODbL credit via AGigLineHUD

Completes "Geographic attribution display" for 0.1.0. The demo is built from
OpenStreetMap data (ODbL 1.0) and needs attribution wherever the derived work
is shown, so the credit now lives in the running game, not just the documents.

- AGigLineHUD (C++): plain AHUD::DrawText in the bottom-left draws
  "(c) OpenStreetMap contributors" and "Data (c) OpenStreetMap contributors,
  ODbL 1.0 - openstreetmap.org/copyright", with a shadow pass so it stays
  legible over bright pavement and sky. No UMG widget asset is involved, so a
  cook cannot drop the attribution through a missing widget reference.
- AGigLineGameModeBase: HUDClass = AGigLineHUD, so the credit is on by default
  in both the DemoMap and the ReviewMap.
- AGigLineShotActor: screenshots are requested with the UI shown, so the credit
  appears in every capture under Screenshots/ - the obligation is provable from
  the images rather than only described.

Docs:
- THIRD_PARTY_NOTICES records that the attribution is displayed in-game and
  where the code lives.
- Screenshots/README documents the overlay.
- TODO marks the item done and narrows the remaining ODbL question to the
  shipped-derivative-database posture.

Squash of branch gameplay-attribution-0.1.0 (PR #4).

Game/GigLine/Source/GigLine/GigLineGameModeBase.cpp

@@ -1,8 +1,11 @@
 #include "GigLineGameModeBase.h"
 
 #include "GigLineCharacter.h"
+#include "GigLineHUD.h"
 
 AGigLineGameModeBase::AGigLineGameModeBase()
 {
 	DefaultPawnClass = AGigLineCharacter::StaticClass();
+	// ODbL attribution must be visible in the running demo, not only in docs.
+	HUDClass = AGigLineHUD::StaticClass();
 }