Support for Sigil II

[mikeday0]

Closes #1090.

[fabiangreffrath]

Thank you for starting to work on this! I really couldn't get myself motivated, but the prospect of ever recurring requests to add support for this wasn't encouraging either.

I guess the one big part that's missing is the special treatment in d_pwad.c and the decision whether to prefer Sigil 1 or 2 for side-loading - or simply load both and live with (or circumvent) the incompatibilities introduced by Sigil 2's DEHACKED lump.

[mikeday0]

I think it's done? A few comments:

• The sideloading system will try to load both Sigil and Sigil II if available.
• The Sigil II WAD with the Thorr soundtrack is loaded in place of the free WAD, not in addition to.
• I opted to add a special case for the Mastermind HP buff, but only when we sideload. If the user explicitly adds the Sigil II PWAD, the dehacked HP change takes effect globally.
• Still using the *_ZD sky. The normal sky texture has that ugly seam. (@JNechaevsky - I thought you added a fix for this, but maybe I'm doing something wrong?)

[JNechaevsky]

I thought you added a fix for this, but maybe I'm doing something wrong?

Nope, nothing wrong! *_ZD sky is a normal one of 256x128 size. In compat version I simply edited TEXTURES/1 lump, so SKY1 is using SKY6_ZD patch (256x128) and the texture itself is also 256x128, that's all the trick.

[JNechaevsky]

Made a brief experiment with brightmapping of SIGILTX2 exit texture. No, nothing, nada. Even if gold eyes will be fine-tuned, there are still few dark-gold pixels on the left side of the texture.

Another problem is esthetical: if gold eyes will be brightmapped, then centered eye will loose it's accent. Brightmapping only centered's eye pupil even if possible will probably looks... Definitely odd. Better leave this texture for good. 🙅🏻‍♀️

[fabiangreffrath]

Very clean, good job. Thanks for this!