Add a Dreamcast GLdc sample that loads a .dt texture from romdisk.
This adds a new nehe06_dt sample based on the existing NeHe cube demo, but loading a Dreamcast .dt texture directly from romdisk instead of the older texture formats.
Changes:
- add samples/nehe06_dt/main.c
- add samples/nehe06_dt/romdisk/NeHe.dt
- register the sample in CMakeLists.txt
The sample is useful for checking GLdc texture loading with Dreamcast-native .dt assets and for comparing the output against the source artwork on hardware.
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