Fixed native iOS preview issue

[sauwming]

Fixed a couple of issues with iOS native video preview:

1. Currently the iOS native preview will be added to the renderer view, which may cause a combined display between local preview and remote video. With the patch, native preview and renderer view will be separated. The native preview view can be obtained using the capability PJMEDIA_VID_DEV_CAP_INPUT_PREVIEW while the remote video is obtained using PJMEDIA_VID_DEV_CAP_OUTPUT_WINDOW as before.
2. Fixed a bug in pjsua_vid which will still connect local preview to the renderer slot even if the preview is native, which also causes mixed display. And since the mixing is done by video conference, the result is inseparable (while for the issue 1, you can still extract the layers with some effort).

The sample app ipjsua will be updated to give an example on how to display the local preview.

Note that the issue should not affect applications that use OpenGL renderer (which is the default) instead of UIView, or if they do not use the native preview feature.