Allow accessing pointers to fields in struct aliases

[keynmol]

Consider:

  opaque type RenderTexture = CStruct3[CUnsignedInt, Texture, Texture]
  object RenderTexture:
    given _tag: Tag[RenderTexture] = Tag.materializeCStruct3Tag[CUnsignedInt, Texture, Texture]
    def apply()(using Zone): Ptr[RenderTexture] = scala.scalanative.unsafe.alloc[RenderTexture](1)
    def apply(id : CUnsignedInt, texture : Texture, depth : Texture)(using Zone): Ptr[RenderTexture] = 
      val ____ptr = apply()
      (!____ptr).id = id
      (!____ptr).texture = texture
      (!____ptr).depth = depth
      ____ptr
    extension (struct: RenderTexture)
      def id : CUnsignedInt = struct._1
      def id_=(value: CUnsignedInt): Unit = !struct.at1 = value
      def texture : Texture = struct._2
      def texture_=(value: Texture): Unit = !struct.at2 = value
      def depth : Texture = struct._3
      def depth_=(value: Texture): Unit = !struct.at3 = value

There is no safe way to get Ptr[Texture] from RenderTexture