Use the wlroots gles2 renderer instead of our own renderer

[ErikReider]

Replaces our renderer with the generic wlroots gles2 renderer while only keeping what we need.

This will allow us to add additional renderers in the future (like vulkan) without soft-forking the entire wlroots rendering logic.

There's one breaking change which is the addition of scenefx_init() which replaces fx_renderer_create()

To test with SwayFX, use this PR

We could go a step further and make the fx_renderer generic like the base wlr_renderer struct, creating a fx_gles2_renderer struct. This would "simplify" the process of adding a new renderer even further.

[WillPower3309]

I'll review what we have so far, but what's pending?