Fix ImGui::NewFrame throws assertion failure in VM with VSYNC+waiting disabled about opensim-creator HOT 2 CLOSED

After a bit of digging around with "this should never happen"-style assertions I found that the bug was because SDL_GetPerformanceCounter can sometimes return the same value twice, rather than monotonically increasing. This then later causes a DeltaTime of 0 to slip into the backend and trigger the assertion.

I wrote a quick hotfix for this that adds a single tick to the performance counter whenever this happens, so that, from ImGui's perspective, time has moved forward (by an insanely small amount) and, when the counter starts increasing again, the mitigation is ignored.

Upstream here: ocornut/imgui#6189

from opensim-creator.

The hotfix seems to stabilize the UI in the VM, so I'll close this (as "hotfixed") and separately handle any bugs related to the hotfix, updates from ImGui upstream, etc. in separate issues.

from opensim-creator.