Update ImGui to v1.86

[mworchel]

First of all, thanks for the great framework!

I noticed that the version of the ImGui dependency is rather dated (April 2020) and incompatible with some awesome libraries like ImPlot (https://github.com/epezent/implot).

So, this PR updates the ImGui submodule from v1.76 to v1.86.

These are the "major" changes:

1. ImGui backends are now located under imgui/backends instead of imgui/examples
2. ImGuiCol_ModalWindowDarkening was renamed to ImGuiCol_ModalWindowDimBg
3. The Columns API is deprecated but still functional in the file imgui_tables.cpp
4. The power argument to ImGui::SliderFloat/ImGui::DragFloat has been replaced by a flags argument (see API breaking change in 1.78: Slider/Drag final "float power = 1.0f" arg becomes flags ocornut/imgui#3361). I replaced all appearances of power > 1 with the flags ImGuiSliderFlags_Logarithmic | ImGuiSliderFlags_NoRoundToFormat, which I think results in natural behavior for the logarithmic sliders. Feel free to double check.

All tests are passing on my machine.

[nmwsharp]

Thank you!! This looks good to me once we can get the CI checks passing.

It looks like compilation with the openGL-less "mock" backend is failing (but only on OSX for some reason?). I just took a really quick glance, but it's not clear ot me what's going on. The failure is in imgui_impl_opengl3.cpp, but I don't think that file should even be built on the mock backend. Perhaps some CMake problems? Do you see the issue?

[mworchel]

Hm, as far as I can tell, the workflow build compiles with both the mock backend and the glfw backend (https://github.com/nmwsharp/polyscope/blob/master/.github/workflows/macos.yml#L19). Is this intended?

Unfortunately, I don't have a MacOS system to check if this is only a CI problem or if the build also fails locally. Maybe it is an issue on imgui's side with the OpenGL 3 compatibility for MacOS? (Isn't OpenGL also deprecated on MacOS?)

[nmwsharp]

Ah you're totally right, I forgot that we build with both backends in CI but only test with mock.

I just tried on a local OSX machine, and sure enough it was broken there too. Turns out new imgui version changed how the IMGUI_IMPL_OPENGL_LOADER_CUSTOM define works somehow, but now the library seems to build without it, so I just removed it. I think it's related to the new glfw backend file.

Also, I noticed the source file list changes needed to be propagated to the mock backend as well, so I went ahead and did that. I'll merge once the build passes.

[nmwsharp]

Merged. Thanks again for submitting!

[mworchel]

Awesome! Thanks for looking into it and applying the required fixes. Glad everything works now :)