• engine: fix PCSS soft shadows of point and spot lights using the directional light's bulb radius
• engine: fix a shader program leak when rendering PCSS or VSM shadows with the program cache
  disabled
• vulkan: with the backend.vulkan.enable_pipeline_cache_persistence feature flag, the backend
  keeps its VkPipelineCache across runs through Platform::setBlobFunc, as the GL backend does
  for program binaries; the blob functions must be set before the Engine is created
• materials: the new retroreflection material property can be used in the lit (standard) shading
  model to create retroreflective surfaces
• gltf: KHR_materials_retroreflection is now supported