D3 used to support Aureal 3-Dimensional sound cards, providing 3D spatial audio. The hardware and drivers is outdated, so all support in code has been removed, including UI mixer setting.
Read more about A3D here: https://en.wikipedia.org/wiki/Aureal_Semiconductor#A3D
Join the license header with historical comments using a separator so IDEs can correctly parse the initial header.
Also use .gitattributes to ensure all files are LF.
The MACINTOSH define refers to MacOS Classic (not OS X) which we do not
plan to support. Rather than carry the cruft forever, let's delete it.
NOTE: legacy/ is unused but we're keeping it around, so MACINTOSH uses
there are left alone.
Process used for this commit:
```
git rm -r mac
find . \( -iname "*.c" -or -iname "*.cpp" -or -iname "*.h" \) \
-exec unifdef -UMACINTOSH -o {} {} \;
git restore legacy
git add .
```
Test Plan:
On Mac, build both Debug and Release
```
cmake --build --preset mac --config Debug
cmake --build --preset mac --config Release
```
