Add example to visualize only rotation for a selected device.

[lubosz]

For debugging rotational tracking, this example is quite handy since it does not render VR graphics, but just visualizes the IMU with 3 axes. I wrote it to debug the VIVE controller IMU. It uses the model matrix patch from @ChristophHaag opengl3 branch and is written in GL3 as well, but is very minimalistic. It uses the MIT licensed math_3d.h header to do the linear algebra required for GL3. This is also on @ChristophHaag's GL3 branch, but I created a ext_deps folder in examples, so multiple examples can share it. I added it to meson and CMake. For meson it builds as soon the dependencies are available, for CMake it builds when the SDL examples are enabled.

[Wallbraker]

This looks good to me.

[TheOnlyJoey]

@lubosz fix meson plx