Add SignedByte and SignedShort enum variants

[weiji14]

Support SignedByte (i8) and SignedShort (i16) types by:

• Adding tiff::decoder::ifd::Value::SignedByte and tiff::decoder::ifd::Value::SignedShort enum variants
• Adding TiffFormatError::SignedByteExpected and TiffFormatError::SignedShortExpected enum variants
• Adding two new into_i8 and into_i16 methods under impl Value in src/decoder/ifd.rs

Also included two new TIFF files to test roundtrip decoding/encoding:

• tests/images/minisblack-1c-i8b.tiff
  • Created using gdal_translate -ot Int8 -co PIXELTYPE=SIGNEDBYTE -scale 0 255 \-128 128 tests/images/minisblack-1c-8b.tiff tests/images/minisblack-1c-i8b.tiff
• tests/images/minisblack-1c-i16b.tiff
  • Created using gdal_translate -ot Int16 -co PIXELTYPE=SIGNEDBYTE -scale 0 65536 \-32767 32767 tests/images/minisblack-1c-16b.tiff tests/images/minisblack-1c-i16b.tiff

References:

• Page 16 in https://download.osgeo.org/libtiff/doc/TIFF6.pdf

Addresses #204 (comment)

[fintelia]

Shouldn't there be more logic to actually handle reading/writing these types?

[weiji14]

Shouldn't there be more logic to actually handle reading/writing these types?

I've added a couple of roundtrip tests to check reading/writing SByte and SShort TIFFs, see if that looks ok.

Btw, I looked under https://github.com/libsdl-org/libtiff/tree/v4.6.0/test/images and couldn't find any obvious example TIFF files of int8/int16 types, so I just converted the uint8/uint16 tiffs using gdal_translate with some scaling. Happy to exchange those to better samples if needed.

[weiji14]

Gentle ping @fintelia to see if this PR looks ok? I'm slowly refactoring the geotiff crate downstream to rely on image-tiff's enums, and it would be nice to have this PR wrapped up so that I can continue with georust/geotiff#15.