Copy-paste ImGui_ImplOpenGL2_RenderDrawData from ImGui's example

[eliasdaler]

Looks like it does almost everything that we need and properly sets up/restores states.
Maybe this will allow us to not use resetGLStates anymore?

Here's one problems, though - it doesn't support OpenGL ES. @Alia5 - can you take a look at it and maybe even add ES support via PR to ImGui directly?

[eliasdaler]

I think I'll also attempt to get this function from ImGui's examples folder, and while it might seem shady, the example's implementation is more flexible and are better maintained.

Though it might make build a bit harder for people not using CMake (will need to include one more .cpp file into build)

[eliasdaler]

See this branch where this is now done: /~https://github.com/eliasdaler/imgui-sfml/tree/opengl-sync
Please test it, but it looks good for me.

[eliasdaler]

Looks like with some work it's possible to use this impl directly, but I'm not sure if it'll work on OpenGL ES since the current ImGui-SFML implementation has some #ifdefs in rendering code related to it.

[eliasdaler]

Did this in 224fe6c