Prevent crash when viewing heightmap collision

[iche033]

🦟 Bug fix

Summary

If you try to view heightmap collisions (View > Collisions), the GUI crashes. This PR prints out a warning to say that it's not currently supported instead of just crashing.

The problem is that the heightmaps shaders currently expect texture maps exist (it does not work with just simple diffuse colors). If they are not available, the shaders fail to compile at runtime and cause the GUI to crash.

Note: I accidentally pushed to gz-sim7 and created a reverse commit (3f80b37) to undo the changes 🙇.

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 #2097 (5c0fd12) into gz-sim7 (3f80b37) will not change coverage.
Report is 1 commits behind head on gz-sim7.
The diff coverage is n/a.

❗ Current head 5c0fd12 differs from pull request most recent head da0e835. Consider uploading reports for the commit da0e835 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim7    #2097   +/-   ##
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  Coverage    64.96%   64.96%           
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  Files          356      356           
  Lines        28791    28791           
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  Hits         18704    18704           
  Misses       10087    10087