FLI/FLC _accept: Check for 8-bit colour depth

[vpv-csc]

Fixes #9168 .

Changes proposed in this pull request:

• Standard FLI/FLC files always have 8-bit color depth. This change helps avoid erroneously detecting some TIFFs as FLI/FLC files.
• Update test files Tests/images/timeout-9139147ce93e20eb14088fe238e541443ffd64b3.fli and Tests/images/timeout-bff0a9dc7243a8e6ede2408d2ffa6a9964698b87.fli to claim to have 8-bit colour depth so that the tests related to them will still work.

[radarhere]

Just to link to documentation, https://www.fileformat.info/format/fli/egff.htm describes the header as

typedef struct _FlicHeader
{
  DWORD FileSize;       /* Total size of file */
  WORD  FileId;         /* File format indicator */
  WORD  NumberOfFrames; /* Total number of frames */
  WORD  Width;          /* Screen width in pixels */
  WORD  Height;         /* Screen height in pixels */
  WORD  PixelDepth;     /* Number of bits per pixel */
...
PixelDepth indicates the number of bits per pixel; the value of this field is always 08h.

[radarhere]

Under 'Frequent errors', https://www.compuphase.com/flic.htm lists 'The colour depth is set to zero'. Is it worth also accepting zero as the value?

[radarhere]

Suggested change

	and i16(prefix, 12) == 8 # 8-bit colour
	and i16(prefix, 12) == [0, 8] # depth

[vpv-csc]

Is it worth also accepting zero as the value?

The file magic accepts only 8. Based on that, I'd say no, but I'll leave the decision to you (the maintainers).

We have received two ”false positive” TIFFs from our users. One of them has 0 as byte 12 and one of them has 1. Our problem would not then be completely fixed. But hypothetically there could be a similar TIFF file, which had 8 as byte 12, as well. With the 16-byte limit of prefix this can never be perfect.

[radarhere]

I've created #9183 to also help the issue. If you don't feel that changing _accept is sufficient, then is that better than this? Or would you like both PRs?

[vpv-csc]

I concentrated too much on _accept when trying to find a solution. #9183 actually fixes this issue as well, for both of our test files. This PR is not absolutely necessary. If you are worried about the 0 vs. 8 case, then I guess you can drop this PR.