i-exr: read openexr image files

this reads .exr files if they are 3 channel half float encoded.

parameters

filename the name of the input file, potentially containing %04d for sequences
startid the index of the first file if loading a sequence
noise a the gaussian portion of the noise model
noise b the poissonian portion of the noise model
prim the primaries used to encode this file
trc the tone response curve applied to the data in this file (this will be undone to go to linear in the colour module)