Fixed DirectShow video capture issues.

1. Set correct value of sample size when calling IAMStreamConfig::SetFormat function. For non-RGB media type it can be set to zero value.
(See http://msdn.microsoft.com/en-us/library/windows/desktop/dd373477%28v=vs.85%29.aspx)
This fixes a bug reported in #3864.
2. Fixed media type GUID for I420 format. FOURCC code bytes should be placed in the reverse order.