Cycles: bake transparent shadows for hair

These transparent shadows can be expansive to evaluate. Especially on the
GPU they can lead to poor occupancy when only some pixels require many kernel
launches to trace and evaluate many layers of transparency.

Baked transparency allows tracing a single ray in many cases by accumulating
the throughput directly in the intersection program without recording hits
or evaluating shaders. Transparency is baked at curve vertices and
interpolated, for most shaders this will look practically the same as actual
shader evaluation.

Fixes T91428, performance regression with spring demo file due to transparent
hair, and makes it render significantly faster than Blender 2.93.

Differential Revision: https://developer.blender.org/D12880

intern/cycles/render/attribute.cpp

@@ -366,6 +366,8 @@ const char *Attribute::standard_name(AttributeStandard std)
       return "pointiness";
     case ATTR_STD_RANDOM_PER_ISLAND:
       return "random_per_island";
+    case ATTR_STD_SHADOW_TRANSPARENCY:
+      return "shadow_transparency";
     case ATTR_STD_NOT_FOUND:
     case ATTR_STD_NONE:
     case ATTR_STD_NUM:
@@ -603,6 +605,9 @@ Attribute *AttributeSet::add(AttributeStandard std, ustring name)
       case ATTR_STD_RANDOM_PER_ISLAND:
         attr = add(name, TypeDesc::TypeFloat, ATTR_ELEMENT_FACE);
         break;
+      case ATTR_STD_SHADOW_TRANSPARENCY:
+        attr = add(name, TypeDesc::TypeFloat, ATTR_ELEMENT_CURVE_KEY);
+        break;
       default:
         assert(0);
         break;