render3d: a glTF material's factors are drawn - base colour, roughness, metallic and emission

gltf_factors reads pbrMetallicRoughness.baseColorFactor, metallicFactor, roughnessFactor and the
emissiveFactor into the primitive (pr.fac), and prim_factors hands them to every program that draws its
textures (actors, the scatter's meshes and levels, the impostor bake) as 1 - factor, so a program never
given them - or a material that gives none (pr.fac null) - multiplies by 1 and draws as before. An
untextured material with a colour (or a metal/roughness) samples a pure white for it, so the factor is
exactly what it draws: horse_cornea is its own colour, not the 200 grey every untextured part had.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/env.ludic

@@ -836,6 +836,7 @@ export state Render3dState {
   tb_key: string = ""
   tb_version: int = 0
   tb_shadow_ok: bool = false         # the shadow came from its bake this map
+  gltf_pure: int = 0                # a 1x1 white a factor multiplies exactly (gltf_factors)
   tt_dir: string = ""               # terrain_tiles_to: where a map's tiles are written ("" keeps the whole copies)
   tt_key: string = ""
   tt_file: pointer = null           # the tiles' file, open for reading (terrain_tiles.ludic)