This issue tracks the various other issues related to optimizing environment map generation

• Cache generated environment map #336 Cache environment map
• Build or adapt tool for generating PMREM ahead of time #341 Generate PMREM ahead of time
• Support for generating PMREM at runtime off of the main thread #342 Generate PMREM off main thread
• Generate and apply PMREM lazily, and progressively enhance #343 Lazily generate PMREM
• generateDefaultEnvironmentMap() called excessively #344 Don't generate maps more than necessary
• Make PMREM the default when using an HDR environment #346 Pick a better name for experimental-pmrem