Update visual defaults

Somewhat more conventional visual presentation:

+ Use a neutral gray background a-la most other CAD/modelling software
  for better contrast with the geometry (which defaults to a bright
  yellow).  This is done with the "skybox-environment" image in
  model-viewer, so add a new setting value in settings.ts for this (it
  can be overriden in the URL just like other settings)

+ But using a skybox will cause that image to be used for lighting
  too, which is clearly not desired.  So fetch a nice professional
  HDRI image from Polyhaven for lighting.  This is much better (more
  directional, higher contrast) than the default light environment
  anyway.

+ The checkerboard texture isn't really a good default.  Use a 1x1
  white pixel instead, essentially presenting the model materials
  unchanged.

Also collect the default color in gltf.py out of the code and put it
next to the texture for clarity.  This should probably be wired
through to a setting at some point.

Signed-off-by: Andy Ross <andy@plausible.org>

frontend/viewer/ModelViewerWrapper.vue

@@ -215,7 +215,7 @@ watch(disableTap, (newDisableTap) => {
   <model-viewer ref="elem" v-if="sett != null" :ar="sett.arModes.length > 0" :ar-modes="sett.arModes"
                 :environment-image="sett.background" :exposure="sett.exposure" :autoplay="sett.autoplay"
                 :orbit-sensitivity="sett.orbitSensitivity" :pan-sensitivity="sett.panSensitivity"
-                :poster="poster" :shadow-intensity="sett.shadowIntensity" :skybox-image="sett.background"
+                :poster="poster" :shadow-intensity="sett.shadowIntensity" :skybox-image="sett.skybox"
                 :src="props.src" :zoom-sensitivity="sett.zoomSensitivity" alt="The 3D model(s)" camera-controls
                 camera-orbit="30deg 75deg auto" interaction-prompt="none" max-camera-orbit="Infinity 180deg auto"
                 min-camera-orbit="-Infinity 0deg 5%" style="width: 100%; height: 100%">
@@ -302,4 +302,4 @@ watch(disableTap, (newDisableTap) => {
   float: left;
   transition: width 0.3s;
 }
-</style>
+</style>