highgui: fix segfault on CvCapture_GStreamer::retrieveFrame

CvCapture_GStreamer::retrieveFrame assumes that RGB videos are 24BPP.
This is not necesarily the case, unless we explicitly tell GStreamer
that we want 24BPP RGB streams.

Adding bpp=(int)24 to the appsink caps.

Here is a video that causes a segfault without this change: 
  http://vclab.science.uoit.ca/~luis/pigbloodtests/2011.09.29.1/wolverine_drop_1.avi
If GStreamer produces a buffer smaller than 24bpp, the image will be much larger than the buffer, leading to a segmentation fault when trying to read beyond the buffer.