Lighting and shadows

Physically based shading in the manner of URP's Lit for Standard, URP Lit and glTF materials:
- metallic and smoothness, a GGX highlight, and emission (Standard's needs _EMISSION, as in Unity);
- ambient from a sky/ground hemisphere, with reflections of it that blur with roughness.
- Legacy diffuse shaders stay Lambert.
Up to eight point and spot lights (the brightest), with URP's smooth range falloff and spot cones (innerSpotAngle).
The brightest directional light casts shadows:
- one shadow map fitted to each camera's view out to 50 units and snapped to texels, fading at the edge;
- hard or soft (3x3 filtered); shadowStrength, shadowBias, shadowNormalBias;
- cutout materials cast cut shadows; renderers' shadowCastingMode (including shadows only) and receiveShadows count; sprites don't cast.
Unity scenes import lights' shadow settings and renderers' cast-shadows mode.
GPU tests check a shadow on the ground under a slanted light (and none from a non-caster), the PBR highlight and emission, and point light falloff.
Not yet: normal, metallic, occlusion and emission maps; skyboxes and reflection probes; RenderSettings ambient from the scene; shadow cascades; point and spot light shadows; post-processing.
Next: the rest of uGUI's controls (ScrollRect, InputField, Dropdown, masks), and texture maps for PBR materials.