OpenGL support

## Why? and Why Need?
Weak Hardware which doesn't fulfill vulkan neither DirectX12 fully support. Make sure Bevy runs on most hardware by sacrificing new features. And use basis PBR to run with OpenGL backend.

## What's for?
Some of embedded hardware like Raspberry pi 3, Intel Haswell, ...

## Benefits?
Run on older harder to play 2D games atleast, without need to shift WGPU + WINIT lib

> In short: People with potato PC also exist

Start by surveying Bevy's rendering and backend architecture alongside its Vulkan and DirectX12 support; the issue names no files, entry points, or tests. Done means providing an OpenGL backend that can run basic PBR and 2D games on hardware such as Raspberry Pi 3 and Intel Haswell.

Written by the indexing model from the issue text.