Flip heightmap's Y position on Ogre 2 and add sanity checks for NaN

[chapulina]

🦟 Bug fix

• Related to DEM improvements to nodata, buffer and other corner cases gz-common#321

Summary

Added sanity checks for NaNs. Without gazebosim/gz-common#321 ign-common may let some slip in, causing both Ogres to crash.

Also flipped the Y position of the heightmap for Ogre 2. As it can be visible on the example, now Ogre 1 and 2 match, and the coordinates follow the right-hand-rule:

Ogre 1	Ogre 2

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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.

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[codecov]

Codecov Report

Merging #585 (6a3465e) into ign-rendering6 (3b0426e) will decrease coverage by 0.00%.
The diff coverage is 37.50%.

@@                Coverage Diff                 @@
##           ign-rendering6     #585      +/-   ##
==================================================
- Coverage           54.34%   54.34%   -0.01%     
==================================================
  Files                 198      198              
  Lines               20220    20225       +5     
==================================================
+ Hits                10989    10991       +2     
- Misses               9231     9234       +3     

Impacted Files	Coverage Δ
ogre2/src/Ogre2Heightmap.cc	0.00% <0.00%> (ø)
ogre/src/OgreHeightmap.cc	72.68% <75.00%> (-0.02%)	⬇️

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[chapulina]

Retargeted to Fortress and added a note to the migration guide.

[iche033]

looks good to me.