Skip demo broken frames during capturing

[YaLTeR]

playdemo has broken frames in the beginning and during loads. It might be possible to detect them automatically and omit from the video. This could also include 1 frame of menu when doing playdemo demo;bxt_cap_start.

[SmileyAG]

Can it be at least optional, please? Some of broken frames can be used as starting frames if you crop and order them correctly.

Their only lack is the missing of a entities or weapon viewmodel (but I hope that you can somehow make draw that even from first frame of the demo)