Minimal changes needed to make the example build and run on macOS

[floooh]

Fixes to make the example build and run on current Macs:

• all the math functions in implot3d.cpp with std:: prefix throw compilation errors, fixed by adding a #include <cmath> at the top (I wonder though if it is better to just remove the std:: prefix. Let me know if I should try that instead.
• there's one place with the same problem in implot3d_demo.cpp where std::sqrt() is called, here I simply removed the std:: prefix though (since the other math calls in that source file don't use the prefix either)

This was enough to make the example build, but then running failed with an error message that the GL context couldn't be created. This is fixed by changing the GL context version to 3.2 and enabling the GLFW_OPENGL_FORWARD_COMPAT GLFW window hint.

Result:

NOTE: I haven't tested those changes on any other platform.

[floooh]

Ok, I just tested the alternative to not include <cmath> and instead replace the math functions with the Dear ImGui math functions (ImSin, ImCos, ImFabs, ImAcos and ImSqrt).

IMHO this is the better solution. From your comment it sounds like you want to make that change anyway:

#5 (comment)

...once that change lands I can remove the #include <cmath> and rebase the PR on your changes (or let me know if you'd rather want to close this PR and I can provide a fresh one based on your changes to fix the macOS build).

[brenocq]

Thank you for the work @floooh! I think it is ready for merging after cmath is removed

[brenocq]

Amazing! Thanks a lot @floooh 🚀