Add support for OpenGL 2

[barracuda156]

Is your feature request related to a problem? Please describe.
Without GL2 support, the app is unusable on systems without GL3, it crashes on launch.

Describe the solution you'd like
Add/restore support for GL2. imgui supports it already: https://github.com/ocornut/imgui/blob/43a90adc618207da0e9ef6274f1b172c76f220ae/backends/imgui_impl_opengl2.cpp
But these sources are missing in the bundled copy.

Additional context
Probably the failure happens here:

namespace ImPlay {
Window::Window(Config* config) : Player(config) {
  initGLFW();
  window = glfwCreateWindow(1280, 720, PLAYER_NAME, nullptr, nullptr);
  if (window == nullptr) throw std::runtime_error("Failed to create window!");
#ifdef _WIN32
  hwnd = glfwGetWin32Window(window);
  if (SUCCEEDED(OleInitialize(nullptr))) oleOk = true;
#endif

  initGui();
  installCallbacks(window);
  ImGui_ImplGlfw_InitForOpenGL(window, true);
}

And the error is

$ implay -v
GLFW [65543]: The requested client API version is unavailable
Error: Failed to create window!

[barracuda156]

I restored imgui sources and fixed the build against them, but it still fails on initializing mpv gl context.