Support other graphics backend at runtime

[marysaka]

Currently we only support OpenGL 3 as our graphics backend but ImGui can support less restrictive backends.

Supporting other graphics backend could also enable support of older hardware and virtual machines without hardware acceleration (that can typically be found in infosec lab environment for example).

[Calinou]

Supporting OpenGL ES 2.0 would be useful for Raspberry Pi/ARM SBC support as well.