Support HDR (.hdr) images as environment maps

- Added loader for hdr images.
  - Added "Render Settings" control with "exposure" so the final rendering
    can be better controlled.
  - Added a hard-coded tone mapper. We can consider exposing it in render
    settings in the future if we like.
  - Normalized color environment maps (e.g. png) have been tweaked so the
    background image is affected by the exposure setting.
- Fixed redraw related bugs
  - Changing the background image/state would lead to a single rendering in
    which the lighting was incorrect. This was a due to a synchronization
    error between the rendering system and the image loading.
- test/background.html exercises all features.
  - the glTF cube has been replaced with a sphere to better showcase the
    environment maps.
  - The environment maps are now external images (rather than data urls in
    the html).

Author: SeanCurtis-TRI

Readme.md

@@ -586,6 +586,8 @@ The default MeshCat scene comes with a few objects at pre-set paths. You can rep
     <dd>The camera from which the scene is rendered (see below for details)</dd>
     <dt><code>/Background</code></dt>
     <dd>The background texture, with properties for "top_color" and "bottom_color" as well as a boolean "visible".</dd>
+    <dt><code>/Render Settings/<object></dt>
+    <dd>Contains properties for how the overall scene renders in your current session. This is not a 3D element in the scene, but provides parameters for configuring the renderer.</dd>
 </dl>
 
 ### Camera Control