Fix compiler warnings

[Blast545]

Summary

Fixes the warnings present in main jobs of he buildfarm.

Reference build:
https://build.osrfoundation.org/job/gz_gui-ci-main-noble-amd64/5/

There are two warnings being fixed:

1. In MinimalScene plugin, if GZ_GUI_HAVE_VULKAN is not defined, the parameter _camera of the TextureNode constructor is not used. I added a void statement to suppress that warning.

2. In ImageDisplay plugin tests, there are shared_pointers declared with an incompatible new/delete match.
   std::shared_ptr<unsigned char> buffer(new unsigned char[bufferSize]);
   Given that the shared pointer is not declared as an array, the tests leak memory as they are right now. I added a fix for all of those tests.

cc: @Crola1702

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)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[Blast545]

AFAIK, gz_gui-ci-pr_any-jammy-amd64 shouldn't be running on gui-main. Given that gz_gui-ci-pr_any-noble-amd64 is green, I will go ahead and merge this PR.

Edit: I can't merge because the check is set as "required" t-t