fix bugs in GIF decoding

[redhecker]

Pull Request Readiness Checklist

this is related to #25691

i solved two bugs here:

1. the decoding setting:
   according to https://www.w3.org/Graphics/GIF/spec-gif89a.txt

    DEFERRED CLEAR CODE IN LZW COMPRESSION

    There has been confusion about where clear codes can be found in the
    data stream.  As the specification says, they may appear at anytime.  There
    is not a requirement to send a clear code when the string table is full.

    It is the encoder's decision as to when the table should be cleared.  When
    the table is full, the encoder can chose to use the table as is, making no
    changes to it until the encoder chooses to clear it.  The encoder during
    this time sends out codes that are of the maximum Code Size.

    As we can see from the above, when the decoder's table is full, it must
    not change the table until a clear code is received.  The Code Size is that
    of the maximum Code Size.  Processing other than this is done normally.

    Because of a large base of decoders that do not handle the decompression in
    this manner, we ask developers of GIF encoding software to NOT implement
    this feature until at least January 1991 and later if they see that their
    particular market is not ready for it.  This will give developers of GIF
    decoding software time to implement this feature and to get it into the
    hands of their clients before the decoders start "breaking" on the new
    GIF's.  It is not required that encoders change their software to take
    advantage of the deferred clear code, but it is for decoders.

at first i didn't consider this case, thus leads to a bug discussed in #25691. the changes made in function lzwDecode() is aiming at solving this.

2. the fetch method of loopCount:
   in the codes at https://github.com/opencv/opencv/blob/4.x/modules/imgcodecs/src/grfmt_gif.cpp#L410, if the branch is taken, 3 more bytes will be taken, leading to unpredictable behavior.

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[asmorkalov]

Why not lzwTableSize?

[redhecker]

because (1<<12) is the maximum size of lzwtable, thus i use (1<<12)+1 here

[asmorkalov]

@sturkmen72 Could you take a look too?

[redhecker]

i add some warning logs for unexpected cases and turn 1<<12 to a const value

[sturkmen72]

@redhecker thank you. @asmorkalov Looks perfect to me.

[asmorkalov]

Run cd /home/ci/opencv
modules/imgcodecs/src/grfmt_gif.cpp:347: trailing whitespace.
+            //    * notice that if the lzw table size is full, 
Error: Process completed with exit code 2.