fix: use correct RGB shifts on big endian

[zeldin]

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Description:

This fixes incorrect colors in DearPyGui when running on big-endian systems. Since e.g. GL_RGBA means that R is in the "first" bits, this means the least significant bits on little-endian and the most significant bits on big-endian, so the shifts need to be different.

Tested on ppc64 (big endian) and aarch64 (little endian).

Concerning Areas:

[Pcothren]

https://github.com/ocornut/imgui/blob/467a1cd4a583edd55db310c82e2823692616e108/imgui.h#L2326-L2327
probably shouldn't put it in imgui.h

[zeldin]

@Pcothren Do you mean setting the shifts in imconfig.h instead? It seems more logical to me to fix the broken defaults instead of overriding them, but either way works I guess. Or are you talking about a refactoring to remove all the IM_COL32 stuff from imgui.h completely?

[Pcothren]

Any changes in imgui.h would be overwritten when we pull in the newer version of imgui.h

[zeldin]

Well, wouldn't the correct way to deal with that be to upstream the bugfix to whereever you are pulling new versions from?
If you were thinking about overriding the defines in the IMGUI_USER_CONFIG file, it doesn't seem like it would work with the imgui.h you have now since the defines are unconditionally defined (and used in inline functions) after including the user config file. So you'd need a change in imgui.h anyway...