Allow to embed korge inside compose in several platforms

It is already posible to do so in Android.
We should cover cases where Metal is used instead. We can render with opengl to a metal texture for now in those scenarios, and then eventually use the metal backend.

Start by locating the existing Android Compose embedding and reviewing how platform rendering handles Metal-based targets. The work is done when Korge can be embedded in Compose on the affected platforms using OpenGL rendered to a Metal texture, with the eventual native Metal backend left for later.

Written by the indexing model from the issue text.