RFC: add libdeflate to SuperBuild

The GDAL 3.2.0 release notes mention:

• Add optional libdeflate dependency for faster Deflate compression/decompression

This is the last test case from #2136:

Format	Size	Compression time	Compression time (MT)	Decompression time
Uncompressed	241 187 444 b	N/A	N/A	N/A
DEFLATE (GDAL 3.2.0)	159 937 804 b	2.053s	0.392s	1.722s
DEFLATE (GDAL 3.1.0)	159 740 861 b	6.638s	0.572s	1.311s

There's a huge improvement in DEFLATE performance in 3.2.0.

This might need adding a new library (libdeflate) to SuperBuild and also upgrading libtiff.