This reduces resource utilisation for Views that never need shadows.
It saves a UBO, two Entities and about 10KB memory. We also lazily
allocate the debugging DataSource, which saves about 10K per View
in debug builds.
Overall this change makes "simple" Views less than 4KB heavy down from
about 24KB (debug, 14KB release).
The main changes:
- ShadowMapManager is now allocated lazily
- the ShadowMap cache object is also allocated lazily
- debug DataSource is allocated lazily
- ShadowMaps are prepared/initialized with a Builder, which makes it
clearer that some APIs are only for preparing the ShadowMap cache.
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