The Tonemap node has a wrong luminance scale. This is because the
parallel reduction shader for logarithmic sum had a wrong identity
value. In particular, its identity was set to 0.0, but since its
initialization macro computed the log, the zero becomes a rather large
negative value.
To fix this, the general structure of the parallel reduction shader was
changed such that the identity is used as is, and not passed to the
INITIALIZE or LOAD macros. This simplifies the implementation and even
avoid the extra texel fetches at the boundary.
02fc4d6481