minor compatibility fixes making sure some widgets and examples are working with py3.3+ and 2.7.x
py3: try to get gi works on gst buffer
gstreamer use mem.get_memory(0) to get correct address
video_gstreamer: use Gbuffer size and MemoryView size to ascertain the
offset for video
Merge pull request #1367 from kivy/py3_fixes

Py3 fixes