About reimplementing MeshTexture

[beicause]

After godotengine/godot#102028, I implemented anthoner MeshTexture in script using RenderingDevice rasterization pipeline and Texture2DRD https://github.com/beicause/GodotMeshTextureRd.
In basic test of rendering a large number of meshes, the Texture2DRD+Sprite2D is even a bit faster than MeshInstance2D, while viewport texture+Sprite2D (the previous pr) is about 3x-5x slower. I'm satisfied with the RenderingDevice, except it can't be used in compatibility renderer.
My knowledge of godot is limited, perhaps we can reimplement MeshTexture in RenderingServer level, and it could also support custom shaders (maybe spatial shaders, but this would make it unavailable for 2D projects with 3D disabled).