Render the Environment light's shadow from its image (#2436)

The Casts Shadows option Environment lights got is only half a fix if
nothing shades for it: an image lights the scene from every direction,
and what occludes that light is the image itself — its dominant bright
region, the sun in a sky map.

- dominant_direction finds it: the radiance-weighted mean texel
  direction (solid-angle weighted, rotation applied). A uniform image
  cancels to zero — a flat default environment shines from everywhere
  at once and casts no shadow, which a uniform environment test pins.
- The shadow is one orthographic map over the scene from that
  direction, after the lights' maps, like a parallel light's: PCF
  widened by Shadow Diffusion, darkened by Shadow Darkness, cleared
  when nothing casts, the image is directionless, or the texel budget
  drops it.
- It darkens the environment's own diffuse and specular light on
  Accepts Shadows surfaces and counts in Cast Shadows Only. CPU and
  GPU (WGSL) do the same; Globals carries the environment's sun
  direction and shadow settings (40 → 44 floats, preflight updated).

A bright-disc environment turned to shine from behind the camera now
shadows a sphere onto the floor behind it, Cast Shadows off lights it
evenly again, and the GPU scene's sky gradient takes a shadow-casting
Environment light. Also: the renderer-switch toasts read "...renderer,
which draws 3D models", the dialog's Casts Shadows checkbox gets an
auto id (dialog.light.casts) the tests click, and the docs' Environment
notes cover the shadow.

docs/agents.md

@@ -214,9 +214,11 @@ Things that tripped up a long agent-driven session (a multi-scene 3D piece built
   image that metallic surfaces reflect. Set its `lightOptions/source` to a footage, comp or solid
   layer by layer id; with no source it uses the comp's Environment Layer (`layer.environment`),
   or a flat default environment of the light's colour × intensity when there is none — either
-  way the light brightens the scene, and it has `castsShadows`/`shadowDarkness`/`shadowDiffusion`
-  like the other lights. It only lights Advanced 3D comps, so creating one (or changing a light's
-  type to Environment) switches a Classic 3D comp to Advanced 3D
+  way the light brightens the scene. It has `castsShadows`/`shadowDarkness`/`shadowDiffusion`
+  like the other lights: Cast Shadows shades the scene from the image's dominant bright region
+  (a flat or otherwise uniform environment has none, so it casts no shadow). It only lights
+  Advanced 3D comps, so creating one (or changing a light's type to Environment) switches a
+  Classic 3D comp to Advanced 3D
   ([#261](https://github.com/storytold/effectcraft/issues/261),
   [#2436](https://github.com/storytold/effectcraft/issues/2436)).
 

docs/architecture.md

@@ -161,9 +161,11 @@ light from an Environment light whose Source (or the comp's Environment Layer, L
 Environment Layer, or a flat default environment of the light's colour × intensity when
 there is neither) is an equirectangular image (cosine irradiance map; roughness-indexed mip
 chain with the split-sum environment BRDF fit), and percentage-closer-filtered shadow maps
-(spot: perspective, parallel: orthographic over the scene, point: six cube faces). An
-Environment light only lights Advanced 3D comps, so adding one (or changing a light's type to
-it) switches a Classic 3D comp to Advanced 3D, as model layers do. Cards keep
+(spot: perspective, parallel: orthographic over the scene, point: six cube faces, environment:
+one orthographic from the image's dominant bright region — none when the image is
+directionless, as a flat environment is). An Environment light only lights Advanced 3D comps,
+so adding one (or changing a light's type to it) switches a Classic 3D comp to Advanced 3D,
+as model layers do. Cards keep
 Material Options (Accepts Lights/Shadows, Casts Shadows On/Only, Ambient, Diffuse, Specular);
 models and primitives add Base Color, Metallic, Roughness, Emissive. Rasterisation runs at 2×2
 supersampling with a reversed-Z depth buffer; opaque triangles resolve visibility first, the