DirectShow: avoid unnecessary RGB32 -> BGR32 conversion.

Frames in the RGB32 format are actually using the BGR byte order, no need
to do the conversion.

Task-number: QTBUG-29206
Change-Id: I13527bd9dacc8330df78beb0965b31469c1d7a87
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>