perf(sleepers): the instances of the sleepers and timbers of a track sent to the gpu once, as it's laid, and kept there in a buffer of their own; in each pass the runs of them in sight turned by the modelviews of their cells into the instance buffer of the frame by a compute pass, one dispatch and one barrier a pass, and drawn in one instanced draw for each model, part and level of detail, in place of the cpu turning every instance every frame

gl/streambuffer.h

@@ -34,6 +34,8 @@ namespace gl
 		// bigger one (generation() changes, buffer() tells which to bind); the old one stays until the frame is done with, so
 		// what was written to it can still be bound and drawn from later in the frame
 		GLintptr push(void const *Data, GLsizeiptr Size);
+		// the room for Size bytes in the buffer of the frame, as push() makes it, left for the gpu to write (a compute pass)
+		GLintptr reserve(GLsizeiptr Size);
 
 		GLuint buffer() const
 		{