sixel_frame_resize(): If width and height are unchanged, nothing to do.

[rokuyama]

This significantly reduces floating-point calculations when used with, e.g., w3m.

[t-bltg]

@rokuyama, you might want to check out https://github.com/saitoha/libsixel/issues/154, and open a copy of your PR at https://github.com/libsixel/libsixel.