Size.Zero deleted in 4.9 causing code breaking

[sdcb]

Size.Zero deleted in 4.9, cause code breaking, the workaround is replace Size.Zero into default, it might not make sense to me.

We should add back OpenCvSharp.Size.Zero

[shimat]

Size.Zero was introduced a long time ago to be compatible with System.Drawing.Size.Empty, since there was no default(T) in C# 1.0. But now I think default can satisfy all our needs. Lately I have been focusing on reducing code size as much as possible and do not want to make this change.