stb_image: Fix gif two_back memory address

[cocoa-xu]

Hi, this should be a simple fix: since out is always the starting address of all layers, therefore the starting address of two layers before, two_back, should be calculated as follows,

if (layers >= 2) {
    two_back = out + ((layers - 2) * stride);
}

otherwise, it would cause a segmentation fault.

Testing image test_alpha.gif.zip

[geneotech]

Thank you so much, this actually fixes a crash I had on a large GIF file. This PR should be accepted ASAP.

[cocoa-xu]

Hi @nothings, sorry for the ping here. I noticed that there seems to be more people experiencing this issue, #1504 and #1688, and I believe this PR should have properly addressed the root cause. I'll be happy to write a unit test for it if there's a need for it!

[641i130]

This resolved certain gifs causing seg faults on an application I work on for work. Thank you for your work @cocoa-xu !