Add function for extracting single channel data from an RGB[A] image (backport #706)

[mergify]

🎉 New feature

Related to gazebosim/jetty_demo#3

Summary

Adds a new Image::ChannelData function to help extract a single channel (r, g, b, or a) from an RGB[A] image.

Thanks for profiling data from @azeey, it was found when loading the Jetty demo world a big chunk of time is spend on splitting the metallic and roughness maps of glb meshes. The main cause of slow down is found to be the Image::Pixel function as the AssimpLoader iterates through every pixel in the image. With the new Image::ChannelData function, the AssimpLoader now has a faster and more efficient way of retrieving different channels of the input image.

Note that the perf issue with Image::Pixel function was also noticed in #699 (comment). Some changes were made to speed it up but seems like it's still not efficient. We'll need to revisit this later on.

For the Jetty demo world, the speed up for splitting the channels of an image was found to be >10x. See example timings (seconds):

• Distribution_Warehouse_Ceiling.001_MetallicRoughness
  • before: 1.22474
  • after: 0.113241
• Distribution_Warehouse_Container.003_MetallicRoughness
  • before: 0.301955
  • after: 0.0263694
• Distribution_Warehouse_Column.001_MetallicRoughness
  • before: 0.304193
  • after: 0.025477
• Distribution_Warehouse_Door.001_MetallicRoughness
  • before: 0.323877
  • after: 0.0263867

The load time for the demo world reduced from 42s (with gazebosim/gz-sim#3070) to 30s on my machine.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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)
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---

This is an automatic backport of pull request #706 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   graphics/include/gz/common/Image.hh
	modified:   graphics/src/Image.cc
	modified:   graphics/src/Image_TEST.cc

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   graphics/src/AssimpLoader.cc

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