Scope lifetime of rtw_image::fdata to load func

[hollasch]

The linear floating-point image data stored in the rtw_image::fdata member variable had a lifetime from image load to the class destruction. We don't need the original floating-point data after the image has been loaded.

This change moves the floating-point data buffer inside the load() function. It is released before the load function returns.

Resolves #1723

[Copilot]

Pull request overview

This PR refactors the rtw_image class to improve memory efficiency by reducing the lifetime of the floating-point image data buffer. The fdata member variable is converted from a class-level member to a local variable within the load() function, which is freed immediately after the byte conversion is complete rather than persisting until object destruction.

Key changes:

• Moved fdata from class member to local variable in load() function
• Updated convert_to_bytes() to accept fdata as a parameter instead of accessing it as a member
• Simplified width() and height() methods by removing the fdata == nullptr check

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/TheRestOfYourLife/rtw_stb_image.h	Refactored rtw_image class to scope fdata locally within load(), eliminating class member and updating related methods
src/TheNextWeek/rtw_stb_image.h	Applied identical refactoring to scope fdata locally within load(), matching changes in TheRestOfYourLife version
books/RayTracingTheNextWeek.html	Updated documentation to reflect the code changes in rtw_image class
CHANGELOG.md	Added entry for the fdata lifetime improvement and standardized terminology in v4.0.2 section

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