don't call endFrame() unless beginFrame() was called

only use condition_variable on non-emscripten builds

thermion_dart/native/web/include/ThermionWebApi.h

@@ -8,6 +8,7 @@ extern "C" {
 
 void Thermion_resizeCanvas(int width, int height);
 EMSCRIPTEN_WEBGL_CONTEXT_HANDLE Thermion_createGLContext();
+EMSCRIPTEN_WEBGL_CONTEXT_HANDLE Thermion_getGLContext();
 
 
 #ifdef __cplusplus