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Capture images from Leopard Imaging LI-USB30-M021 camera on Linux

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# M021V4L2: Capture images from Leopard Imaging LI-USB30-M021 camera on Linux in C++ and Python3

The LI-USB30-M021 camera from Leopard Imaging is a fast (up to 90 frames per
second) global-shutter CMOS camera that captures images over USB 3.0.  Because the
camera serves up raw image bytes, getting it to work on Linux using V4L2 (Video
For Linux 2) requires a bit of extra format-conversion work.  With some help
from the folks at Leopard Imaging, I was able to write a few simple APIs for
the camera for people who want to use it on Linux without doing the conversion
themselves.

The C++ and Python3 APIs are intended for OpenCV users who want to be able to
capture images as a Mat object (C++) or NumPy array (Python3).  Because the M021
camera supports three frame sizes (1280 x 720 at 60 FPS; 800 x 460 at 90 FPS;
640x480 at 30 FPS), I've provided a class for each frame size. The capture runs
on its own automatically-launched thread. As the code fragment below shows,
the classes are extremely simple to use:

<pre>
 cap = Capture800x460()

while True:

ret, frame = cap.read()

cv2.imshow('LI-USB30-M021',frame)

if cv2.waitKey(1) & 0xFF == 27:
 break
 </pre>

To run the Python3 demo program, cd to <tt>opencv/python</tt> and do
<pre>
 % sudo python3 setup.py install
 % make cap
</pre>

If you get errors about OpenCV or NumPy not being installed, you should make
sure that they're installed now:

<pre>
 % sudo pip3 install numpy
 % sudo pip3 install opencv-python
</pre>

Because Python2 will not be [supported](https://pythonclock.org/) much longer,
I've switched to Python3 for this project, and will not be supporting Python2.

To run the C++ OpenCV capture demo, cd to opencv/cpp and type make
cap.

Both of these programs do some post-capture color balancing to compensate for
the slightly dimmed / green appearance of the image from the camera.

I've also provided a C API (which is used by the C++ and Python3 code) for
capturing images in YUYV format, along with a demo program (a cut-down version
of Guvcview) that displays the images using GTK and SDL. To run the GTK/SDL
demo, cd to gtksdl and type make run.

Known Issues

<ul>
<li>Programs will occasionally seg-fault on exit.
<li> On ODROID XU4, Python3 version is much slower than C++ version.
<li> On desktop Ubuntu 16.04, an Unable to dequeue buffer error
occurs in the OpenCV C++ examples, and no image is displayed.
</ul>

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