Fix FG-13163 attempt imagecrop on transparent background.

[devnexen]

based on @nielsdos proposal.
patches on the embedded version of libgd. Fixing mismatch color model between imageallocatealpha using gdImageGetTransparent and gdImageCropAuto usage.

thankfully CI does not use external gd flag.

[nielsdos]

After the last comment on that issue (#13163 (comment)) I began to doubt that the idea is right.
I see that im->transparent gets initialized to -1 (0xff'ff'ff'ff) causing the initial mismatch. If the user were to set the transparent color using imagecolortransparent then that value is overwritten and it works correctly.
It makes me wonder whether the initial value should be changed instead.

Also, now we tested with imagefill($gdImage, 0, 0, imagecolorallocatealpha($gdImage, 0, 0, 0, 127));
but do we expect the full image to be cropped when using imagefill($gdImage, 0, 0, imagecolorallocatealpha($gdImage, 0xff, 0xff, 0xff, 127)); ? If yes, then we should use a mask to compare the colors against each other in the transparent case. However, as I said earlier I'm not sure if that's desired as the color information is still there, it's just not drawn.

I find the manual insufficient to understand the desired behaviour.

[devnexen]

There is much incertainty in your second point, no idea too. I ll keep it as draft for now anyway, I would need to test few scenarios.