Fixed selecting media type in DShow video capture.

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)
Fixed media type GUID for I420 format in DShow capture.

FOURCC code bytes should be placed in the reverse order.
Fixed getting CV_CAP_PROP_FOURCC property value

Save selected FCC in order to return proper CV_CAP_PROP_FOURCC property value (currently FCC for RGB24 is always returned).
Merge pull request #3298 from a-wi:DShow_capture