SL-19755 Small tweak to make exposure not start at black.

indra/newview/pipeline.cpp

@@ -1280,7 +1280,9 @@ void LLPipeline::createLUTBuffers()
 
     mExposureMap.allocate(1, 1, GL_R16F);
     mExposureMap.bindTarget();
+    glClearColor(1, 1, 1, 0);
     mExposureMap.clear();
+    glClearColor(0, 0, 0, 0);
     mExposureMap.flush();
 
     mLuminanceMap.allocate(256, 256, GL_R16F, false, LLTexUnit::TT_TEXTURE, LLTexUnit::TMG_AUTO);