add internal babl_parse_double; to avoid using atof

atof is locale dependent; it was being used two places parsing floating point
values from environment variables.
extension: make rgb8 to rgba8 conversion faster

For all pixels but the last one, do a 32bit load/store + 8bit immediate store
instead of byte by byte copying.
extensions: use more integer instructions for float lookups
babl: make malloc implementation stability sanity check non-fatal
Fix crash if failing to normalize path for loading extensions

Can for instance happen when running with "BABL_PATH=something:",
which easily happens in scripts on form BABL_PATH=something:${BABL_PATH}