Add offset and stride to ImPlotGetter plots

[GameChaos]

This allows for downsampling ImPlotGetter plots, like the "Time Series" demo in implot_demo.cpp.

Stride and offset don't feel like the right words though, because instead of being a byte offset/stride, they offset/stride the index instead.

Some code I've used this for (apologies in advance): https://codeberg.org/GameChaos/amogus/src/commit/e176744987894a8938b59c6a6124938b7f4cdde4/code/amogus.cpp#L1582-L1597

[James2022-rgb]

Thank you for this change, I hope this can be merged upstream.