According to the gstreamer docs [1], the GstMessage pointer returned by
gst_bus_pop() is nullable, meaning NULL is a valid return value.

Previously, gst_is_missing_plugin_message would throw an assert when its
message object parameter would fail the GST_IS_MESSAGE macro check,
crashing the entire process (unless running in a try-catch block of course).

Instead of relying on valid messages, check if the message object itself is
valid before passing it to other gstreamer functions.

[1] https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstBus.html#gst-bus-popSigned-off-by: Christopher N. Hesse <raymanfx@gmail.com>

Must have that flag. Otherwise following "doxygen overview" tutorial won't work.

- V4L2_CID_MPEG_VIDEO_H264_VUI_EXT_SAR_HEIGHT
- V4L2_CID_MPEG_VIDEO_H264_VUI_EXT_SAR_WIDTH

declaration order is not fixed

Error message:
Core.java:97: error: illegal forward reference
    Hamming_normType = NORM_HAMMING,

- better handling of enum arguments
- less merge conflicts

- added more debug checks

- can be enable separately if needed
- not stable (due storage I/O processing)