Add support for absolute transform

[spessasus]

Fixes #1391

This PR adds support for absolute value modulator transform.

Also fixes duplicated "to ensure" in a comment.

[derselbst]

Looks good, thanks. A few comments below.

Also, a public interface for setting and getting the new transform field needs to be added as well. I.e.

fluid_mod_set_transform(fluid_mod_t *mod, int flags);
int fluid_mod_get_transform(const fluid_mod_t *mod);

Where flags is an enum representing the two possible values. And with API documentation.

[spessasus]

I've added enums and the checks you wanted. The functions are added too (with the comments), but if there's a separate .md or .xml file with the docs, I didn't find it. And lastly, check my comment regarding new_fluid_mod in your review.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
22.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[derselbst]

I tweaked the docs a bit and renamed the enums to be more consistent / easier to memorize. LGTM now, thanks!