api to skip frames

[yamt]

for games, it's often more important to provide a constant game speed than drawing every single frames.
it would be nicer for wasm4 to provide a mechanism for skipping frames.
especially for graphics-heavy games on slow hardware.

eg. a simple timestamp (maybe in fps) on the shared memory area.

static unsigned int n = 0;

update()
{
  if (TIMESTAMP > n++) {
    /* somehow reduce computation for this frame */
  }
}