[Feature] remove stride and offset from ImPlotSpec

**Stride** and **offset** are attributes of plot data, other members in **ImPlotSpec** are attributes of plot style.
It would be better to move **stride** and **offset** member from **ImPlotSpec** to function args or somewhere else, so that a single **ImPlotSpec** variable can serve as predefined plot style for multiple datasets with different memory layouts, thereby separating data from presentation.