Implement bgfx backend

[AlexvZyl]

We can swop out our direct OpenGL calls with bgfx calls which supports all of the backend types (OpenGL, Vulkan, Metal, etc). This will improve the Renderer performance and cross-platform compatibility.

Currently everything is being done with OpenGL, so we will have to refactor shaders, framebuffers, vertexbuffers etc. This should not take too long and the current Renderer API will be kept the same. We will also still keep GraphicsPrimitivesBuffer so that we can keep using FreeLists.

[AlexvZyl]

@Cullen-Enerdyne I think this will work very nicely. We still have the control over graphics and memory we want and can use bgfx to use any API we want, depending on the platform and without having to change anything. In addition to this, we can see how they render text, textures, etc. and see if we want to incorporate it into our engine. I do not think they use MSDF for text, so I might not use their text renderer.

[AlexvZyl]

This library is awesome, it even supports multi-threading the renderer and order independent transparency. Implementing this alongside our Entities and FreeLists can make for something very nice.

[AlexvZyl]

Minecraft (bedrock) uses this library for rendering :)