Option to set number of frames shown in graph/histogram

[CyberShadow]

Currently this is a static array:

https://github.com/flightlessmango/MangoHud/blob/425b98ae9a21b672d2e50338b52ef7efbe384b91/src/overlay.h#L59

Graphs are then drawn using ARRAY_SIZE of that array.

Would be nice if this limit was configurable instead of hard-coded to 200.

Use case: I'm trying to visually demonstrate a performance issue which manifests as stable high FPS -> ~5 seconds of unstable FPS -> stable low FPS.