Refactoring Texture2D.FromStream for better support for hot-reloading textures

Refactoring the core Texture2D.FromStream method into an extension that reads in the image width, height, and pixel data. With this refactor, games that use FromStream can now hot-reload textures in place by reading a new stream and calling SetData without having to additionally wrap the texture or otherwise dance around to prevent destroying object references.

With all the other stuff going on in FNA I don't know if you want this now or ever, but we're using it ourselves and I wanted to pass it back up in case it's useful to others.