Update custom shaders tutorial

[iche033]

• copied the custom shaders over to the example's own media directory so that demo is self-contained.
• tweaked the depth camera shaders to produce a nicer grayscale appearance for objects in the scene (it used to be all white which does not really show that the shaders are doing anything)
• updated tutorial to explain better the two uses of shaders

Signed-off-by: Ian Chen ichen@osrfoundation.org

[codecov]

Codecov Report

Merging #148 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   52.07%   52.07%           
=======================================
  Files         143      143           
  Lines       13105    13105           
=======================================
  Hits         6824     6824           
  Misses       6281     6281           

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