Merge branch 'vertical-sky-distort2' into 'master'

Proper barrel sky distortion

Skies now use proper barrel distortion in both axes, rather than just the x-axis (i.e. just horizontally) as in all previous SRB2 versions. Credit goes to Nev3r for the original change and idea.

Was going to make an option for a non-barrel distorted sky, assuming barrel distortion is the default, but I just can't quite get it to work in a way that is pleasing to the eyes. So I've not bothered with it here for now, unless someone objects to barrel distorted skies for any reason?

See merge request !35

src/r_plane.c

@@ -709,6 +709,7 @@ void R_DrawPlanes(void)
 					if (dc_yl <= dc_yh)
 					{
 						angle = (pl->viewangle + xtoviewangle[x])>>ANGLETOSKYSHIFT;
+						dc_iscale = FixedMul(skyscale, FINECOSINE(xtoviewangle[x]>>ANGLETOFINESHIFT));
 						dc_x = x;
 						dc_source =
 							R_GetColumn(texturetranslation[skytexture],