Parse OGRE2_RESOURCE_PATH as list of strings

[iche033]

🦟 Bug fix

Fixes gazebosim/gz-cmake#361

Summary

There is a possibility that the OGRE2_RESOURCE_PATH cmake variable can contain multiple paths (strings). This PR updates CMake and source code to parses the variable as a list instead of a string.

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.

[codecov]

Codecov Report

Merging #867 (142bc0c) into gz-rendering7 (9f32fcc) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head 142bc0c differs from pull request most recent head b0c6262. Consider uploading reports for the commit b0c6262 to get more accurate results

@@                Coverage Diff                @@
##           gz-rendering7     #867      +/-   ##
=================================================
- Coverage          77.15%   77.14%   -0.01%     
=================================================
  Files                170      170              
  Lines              14720    14724       +4     
=================================================
+ Hits               11357    11359       +2     
- Misses              3363     3365       +2     

Impacted Files	Coverage Δ
ogre2/src/Ogre2RenderEngine.cc	75.83% <50.00%> (-0.21%)	⬇️