package/libraw: security bump to version 0.20.0

package/libraw: security bump to version 0.20.0

- Fix CVE-2020-15503: LibRaw before 0.20-RC1 lacks a thumbnail size
  range check. This affects decoders/unpack_thumb.cpp,
  postprocessing/mem_image.cpp, and utils/thumb_utils.cpp. For example,
  malloc(sizeof(libraw_processed_image_t)+T.tlength) occurs without
  validating T.tlength.

- zlib is an optional dependency since
  https://github.com/LibRaw/LibRaw/commit/b63f017b063edb5e7091e3952ee20cb4d002edbe

Also update indentation in hash file (two spaces) as well as README.md
hash, no license changes:
 - https://github.com/LibRaw/LibRaw/commit/d1975cb0e055d2bfe58c9d845c9a3e57c346a2f9
 - https://github.com/LibRaw/LibRaw/commit/d38361b76e1a405a25b11165a1ee5495fc899246

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>