Fix bits value for RGB;16N unpackers

[kkopachev]

Fixes #3770 . Fixes #3832.

Changes proposed in this pull request:

Change bits value for RGB;16N unpackers. bits used in TiffDecode.c to calculate row size of a tile in bytes.

[kkopachev]

It was originally introduced in 768668c but I guess bits are not directly used as much as I expected, so it became a problem when newer tiff decode code started relying on bits in #3489

[kkopachev]

Appveyor failure is totally unrelated to the proposed changes.

[radarhere]

Thanks for this. Are you able to add tests, that fail without this PR and pass with it?

[kkopachev]

I can only think about adding reading 16bit Tiff file as a test, but that would be indirect test. Not sure how to test just for bits value. Any suggestions?

[radarhere]

Reading a 16-bit TIFF file and comparing the result to another image should be right - testing just the unpacker entry value sounds rather specific.

[kkopachev]

Added test.

Test image is derived from existing test image Tests/images/tiff_16bit_RGBa.tiff using

gdal_translate -co "TILED=YES" -co COMPRESS=DEFLATE tiff_16bit_RGBa.tiff -b 1 -b 2 -b 3 tiff_16bit_RGB.tiff

[radarhere]

I rebased on master so that AppVeyor could try again. Although Azure is now experiencing problems, at least Travis and AppVeyor are passing now.