Mobile-"ish" emscripten support

Full screen "wasm-game" for viewport compatibility between desktop and
web interfaces. Viewport modes ("fit", "center", and "stretch") should
now work the same way under WASM/SDL and SFML. This should also enable
android or web-for-mobile aspect ratios to be supported more easily.

src/SceneTransition.h

@@ -17,6 +17,8 @@ public:
     TransitionType type = TransitionType::None;
     float duration = 0.0f;
     float elapsed = 0.0f;
+    unsigned int width = 1024;
+    unsigned int height = 768;
     std::string fromScene;
     std::string toScene;