tiff: support 32bit float input

- added support for 32bit float tiff input
- tested with enblend/enfuse 32bit float TIFFs
- improved error checking and factored exit path to a single goto label

Note that houz merged a previous pull request for this, but it seems i messed sufficiently with my github repo that he finally merged nothing back to darktable/master :-D

Please merge again if the intended result was to include this for 1.4. But the consensus in previous pull request seemed to mark this as post 1.4