Adds a new option to the blender export plugin to support
Custom Props. Enabling that allows one to set Custom Properties
under the Object data tab in blender and access those on the
userData property of imported objects.

The ObjectLoader already supports importing JSON with userData.

WebGLRenderer: loadUniformsGeneric() back to if statements. Chrome seems to be continuously deoptimizing the function. Maybe helps with #8220.

Matrix inversion

HDR EnvMap example with high quality, flexible PMREM Generator.

Output encoding

Remove example materials_variations_standard2

- Also linearize memory access in set.

float and half HDR CUbe Maps load and are properly generated.  Something wrong when using RGBE encoding.

# Conflicts:
#	src/renderers/shaders/ShaderLib.js
#	src/renderers/webgl/WebGLPrograms.js