Fix reading pixel values from single channel 16 bit image

[iche033]

Signed-off-by: Ian Chen [email protected]

🦟 Bug fix

Summary

Currently the Image class calls FreeImage_GetPixelIndex to access pixel values of a single channel image. However, this function does not support reading values from 16 bit images. This PR fixes the issue by implement our own PixelIndex function that includes logic for accessing the pixel value of 16 bit image types.

Additional info: This is needed for loading 16 bit heightmaps. Currently it floods the console with Image: Coordinates out of range error msgs. This happens when the Image::MaxColor call is looping through the image pixels and calling FreeImage_GetPixelIndex function that fails on every iteration.

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 #439 (ea4289c) into gz-common5 (2542907) will increase coverage by 0.14%.
The diff coverage is 78.26%.

@@              Coverage Diff               @@
##           gz-common5     #439      +/-   ##
==============================================
+ Coverage       80.31%   80.45%   +0.14%     
==============================================
  Files              85       85              
  Lines            9987    10004      +17     
==============================================
+ Hits             8021     8049      +28     
+ Misses           1966     1955      -11     

Impacted Files	Coverage Δ
graphics/src/Image.cc	81.00% <78.26%> (+4.44%)	⬆️

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