Material System: always write all of a given stage's data

[VReaperV]

Fixes #1696

[slipher]

LGTM

Xonotic heightmaps (heightmap in normal alpha) still don't work in material system, but our stuff is all good now.

[VReaperV]

Xonotic heightmaps (heightmap in normal alpha) still don't work in material system, but our stuff is all good now.

Are Xonotic maps that have those directly usable/is there some dpk with xonotic heightmaps to test this with?

[slipher]

Are Xonotic maps that have those directly usable/is there some dpk with xonotic heightmaps to test this with?

You can download https://dl.unvanquished.net/share/xonotic/release/xonotic-0.8.6.zip and extract xonotic-20230620-maps.pk3 (it's self contained). Then launch with -set fs_legacypaks 1 -set r_dpMaterial 1 -set g_neverEnd 1 and devmap erbium for example.