hotfix(glfwbackend): revert part of #348

[gucio321]

caused ERROR: ImGui_ImplOpenGL3_CreateDeviceObjects: failed to compile fragment shader! With GLSL: #version 130

I've investigated a bit more to justify this:
it causes because giu uses mainthread to call Run() in mainthread.
possibly caused by the fact that CreateWindow might be called in another goroutine than Render loop