Avoid quadratic behavior when validating sprites

[ArneBab]

This patch resolves quadratic behavior (that caused waiting for several minutes) when loading charts with dates on the x-axis where thousands of datapoints lie on the same date-position.

To this end it avoids removing sprites from a list inside a while loop over that same list.

I normally do not use typescript, and the type-specifiers are likely to be overly broad. This change is hand-translated from our javascript patches.