pushTexture
Registers an externally-created WebGLTexture in Emscripten's GL texture table.
The returned id can be passed to Skia GL APIs that expect a numeric texture id, such as org.jetbrains.skia.BackendTexture.makeGL. The texture must belong to the same WebGL context that Skiko is using.
This function only creates the Emscripten table entry. If the returned id is passed to a Skia API that takes ownership of the texture, Skia will delete the GL texture through Emscripten and the table entry will be cleared there. If ownership is not transferred to Skia, call unregisterTexture when the id is no longer needed to avoid leaking the table entry.