Fix particle emitter color range image path warning

[iche033]

🦟 Bug fix

Summary

Running the particle_emitter.sdf world prints the following warning:

[error] [GUI] SetColorRangeImage() error: Texture [materials/textures/fogcolors.png] not found. 

This PR makes sure that the full image path is constructed before gz-rendering creates the particle emitter. Note that this just fixes image path and gets rid of the warning but the gz::rendering::SetColorRangeImage function currently does not work (it has no effect) gazebosim/gz-rendering#902.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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