tiff write_image: rewrite. Use memcpy. Do not overflow
Also set field TIFFTAG_ORIENTATION to ORIENTATION_TOPLEFT.

TIFFTAG_SAMPLESPERPIXEL was set earlier, no reason to set it again.
EXR write_image(): eliminate 3 temp buffers with total size of 3*width*height*sizeof(float)
This is also faster, while image's md5 is still the same (when exif is commented, compression is off and pixel type is float)
EXR: wording changes
EXR: threading

*MUCH* faster on computers with multiple processors.
With all recent changes, EXR export (PIZ, float) time is down from 13 sec to 3 sec for me.