No-op, dimension checkings

[Kagami]

It would probably be a good idea to just copy src into dst if source and destination dimensions are equal; also some fool-proof checkings like w, h != 0 and so on.

[Kagami]

Also:

• Use same coeff matrix if w1=h1 & w2=h2
• No-op if either w1=w2 or h1=h2 (sample only one time)
• Sample rows/cols first depending on the ratio, see AS docs:

AviSynth has completely separate vertical and horizontal resizers. If input is the same as output on one axis, that resizer will be skipped. Which one is called first, is determined by which one has the smallest downscale ratio. This is done to preserve maximum quality, so the second resizer has the best possible picture to work with.