message: support variance
  ospTutorialAsync indicates that it is working
diff: #include "MultiDeviceLoadBalancer.h"
  #include "MultiDevice.h"
  +#include "fb/LocalFB.h"
  #include <rkcommon/tasking/parallel_for.h>
  @@ -128,6 +129,7 @@ void MultiDeviceLoadBalancer::renderTiles(api::MultiDeviceObject *framebuffer,
  //gather individual tiles down to fb 0.
  const auto fbSize = fb0->getNumPixels();
  + LocalFrameBuffer* fb0asLFB = (LocalFrameBuffer*)fb0; //todo: revise when we support other types
  OSPFrameBufferFormat colorBufferFormat = fb0->getColorBufferFormat();
  @@ -137,18 +139,12 @@ void MultiDeviceLoadBalancer::renderTiles(api::MultiDeviceObject *framebuffer,
  FrameBuffer *fbi = (FrameBuffer *)framebuffer->objects[subdeviceID];
  //get a hold of the pixels
  - //TODO: formats taken from LocalFrameBuffer, are these always valid?
  const void *color = fbi->mapBuffer(OSP_FB_COLOR);
  int *colorI = (int*)color;
  float *colorF = (float*)color;
  float *depth = (float*)fbi->mapBuffer(OSP_FB_DEPTH);
  vec3f *normal = (vec3f*)fbi->mapBuffer(OSP_FB_NORMAL);
  vec3f *albedo = (vec3f*)fbi->mapBuffer(OSP_FB_ALBEDO);
  - /*
  - //TODO: ensure LocalFrameBuffer_accumulateTile leaves root with satellites' acc and var
  - vec4f *accum = (vec4f*)fbi->mapBuffer(OSP_FB_ACCUM);
  - vec4f *variance = (vec4f*)fbi->mapBuffer(OSP_FB_VARIANCE);
  - */
  //get a hold of the list of tiles on this subdevice
  const int tilesForSubdevice = numTiles / loadBalancers.size();
  @@ -311,16 +307,17 @@ void MultiDeviceLoadBalancer::renderTiles(api::MultiDeviceObject *framebuffer,
  fb0->setTile(tile);
  + //todo: For now we assume LocalFrameBuffer for rank 0 (see MultiDevice).
  + //When that assumption no longer holds asLFB will be invalid and we'll need to
  + //do better.
  + float tileError = fbi->tileError(tileID);
  + fb0asLFB->tileErrorRegion.update(tile.region.lower / TILE_SIZE, tileError);
  });
  fbi->unmap(color);
  fbi->unmap(depth);
  fbi->unmap(normal);
  fbi->unmap(albedo);
  - /*
  - fbi->unmap(accum);
  - fbi->unmap(variance);
  - */
  });
  }
  