Reuse scene/camera/shaders across all instances of EquirectangularToCubeGenerator, PMREMGenerator, and PMREMCubeUVPacker to eliminate memory leak from accumulating WebGLStates. Fixes #15288.

* Examples: use .texture of input render targets.

* Assign null instead of undefined.

Add high quality rand() function to common.glsl and an example to show off the noise textures. (#8603)

* create procedural glsl and an example to show off the noise textures.

* add to files.js

* adopt noise procedural in PMREMGenerator.

* fix page title per @arose's suggestion.

* more adoption of procedural.glsl

* remove procedural.glsl, add a single line to common.glsl.  Update other files as needed.

* cleanup shaders to use common.glsl

* tabify.

* avoid collapsing into a single function - @WestLangley recommendations

* more adoption of tabs.

* fix tabs.

* fix bug where gammaInput/gammaOutput were reset by the PMREM generators.

# Conflicts:
#	examples/js/pmrem/PMREMCubeUVPacker.js

* propagate encodings properly between PMREM CubeUVPacker and PMREMGenerator as well as through texture constructors.

# Conflicts:
#	examples/js/pmrem/PMREMCubeUVPacker.js

* turn off tonemapping, fix None HDR map option.

# Conflicts:
#	examples/webgl_materials_envmaps_hdr.html

* minor envmaps_hdr improvements.

# Conflicts:
#	examples/webgl_materials_envmaps_hdr.html

* fix double decode.

* recalc materials when changing texture encodings.

# Conflicts:
#	examples/webgl_materials_envmaps_hdr.html

* clean up envmap_hdr example.

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

cube_uv_reflection map shader.  (written by @spidersharma03, modified by @bhouston)

clean up PMREMGEnerator, incorporate global encodings.glsl.  Add into the THREE namespace, move into examples.