Adding post-processing support

[adriengivry]

We should support adding custom post-processing into our scenes.

This will require to improve OvRendering to provide a proper abstraction layer for Framebuffer and RenderTexture

[yc-feej]

Are we open to PR right now on this issue? I've implemented shader based post processing effects on alike engine with abstraction as simple as (not exactly the same):

renderer->BeginPostProcess();
renderer->SetPostProcessParameters(&post_process_config_);
renderer->BindPostProcessBuffer();
renderer->RenderPostProcessByFBO(fbo_i
d_);
renderer->UnbindPostProcessBuffer();
renderer->EndPostProcess();

Combined with a reflection system, the parameters could be reflected to the editor as well so that all things are automatic. Even without this, it's still flexible enough to play around with.

A result showing fish eye lens with kannala_brandt model is like this:

If you find this interesting I'll take some time to implement it in Overload.

[adriengivry]

@yc-feej yeah for sure! We are reviewing and integrating contributions! You can definitely participate on this one

[adriengivry]

Addressed by #330