Suggestions to #170

[chapulina]

Refactoring suggestions so we keep all the screnshot logic within the Screenshot plugin. This way:

• There's no need to add any code to the scene, so no screenshot functionality is loaded unless the user wants to use it
• The same screenshot plugin is usable with ign-gui and ign-gazebo (needs Also use Ignition GUI render event gz-sim#598)

The tutorials introduced in gazebosim/gz-sim#596 provide more context on this refactoring.

[codecov]

Codecov Report

Merging #171 (3d9aa76) into jennuine/screenshot (af13052) will increase coverage by 0.43%.
The diff coverage is n/a.

@@                   Coverage Diff                   @@
##           jennuine/screenshot     #171      +/-   ##
=======================================================
+ Coverage                59.88%   60.31%   +0.43%     
=======================================================
  Files                       20       20              
  Lines                     2535     2510      -25     
=======================================================
- Hits                      1518     1514       -4     
+ Misses                    1017      996      -21     

Impacted Files	Coverage Δ
src/plugins/scene3d/Scene3D.cc	12.78% <ø> (-0.13%)	⬇️

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[jennuine]

Works and LGTM! Thank you @chapulina!