integrate and link lib curl for android, add VAO support for brush draw, sync canvas with screen redraw instead of input events

engine/app.cpp

@@ -487,6 +487,10 @@ void App::init()
 
 void App::update(float dt)
 {
+    // update offscreen stuff
+    if (canvas && canvas->m_canvas)
+        canvas->m_canvas->stroke_draw();
+
     glClearColor(.1f, .1f, .1f, 1.f);
     glViewport(0, 0, (GLsizei)width, (GLsizei)height);
     glClear(GL_COLOR_BUFFER_BIT);