[jafin]

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Description

Missing BitsPerSample

• Problem: TIFF 6.0 says that when BitsPerSample is absent, each sample has 1 bit. Bilevel fax and scanner files often leave the entry out. TiffFrameMetadata.Parse kept the 8-bit RGB default in this case. An uncompressed bilevel file without the entry then failed with "The number of samples in the TIFF BitsPerSample entry is not supported". A CCITT Group 4 file decoded, but its frame metadata reported no valid bits per pixel.
• Change: when the entry is absent, the decoder now uses 1 bit for each of the SamplesPerPixel samples (SamplesPerPixel defaults to 1, and TiffBitsPerSample holds at most four samples). An entry that is present but invalid keeps its current handling.

Testing

TiffDecoder_CanDecode_BilevelWithoutBitsPerSample encodes a 16x8 bilevel image, uncompressed and with CCITT Group 4. It then moves the BitsPerSample entry to an unused private tag. The test checks that:

• the pixels match the decode of the unmodified file exactly;
• the frame metadata reports Bit1;
• Image.Identify reports 1 bit per pixel.

All 812 tests in the Tiff area pass in Release on net10.0.

Fixes #3242