use Module.createContext for 2D rendering in emscripten

src/video/emscripten/SDL_emscriptenframebuffer.c

@@ -76,7 +76,7 @@ int Emscripten_UpdateWindowFramebuffer(_THIS, SDL_Window * window, const SDL_Rec
         if (!Module['SDL2']) Module['SDL2'] = {};
         var SDL2 = Module['SDL2'];
         if (SDL2.ctxCanvas !== Module['canvas']) {
-            SDL2.ctx = Module['canvas'].getContext('2d');
+            SDL2.ctx = Module['createContext'](Module['canvas'], false, true);
             SDL2.ctxCanvas = Module['canvas'];
         }
         if (SDL2.w !== w || SDL2.h !== h || SDL2.imageCtx !== SDL2.ctx) {