Add slider to change crosshair size and a core option for the Libretro core

[bslenul]

For standalone I made a slider going from 10 to 100 with 40 being the default, the slider will be greyed out until at least 1 "Crosshair" box is checked:

xhair_size.mp4

For Libretro I made a scaling option going from 50% to 300%:

lr-xhair_size.mp4

For standalone code I wasn't sure how to handle

constexpr int XHAIR_WIDTH = 40;
constexpr int XHAIR_HEIGHT = 40;

So I simply replaced every instances of these with config::CrosshairSize, I hope this is fine... If this is not OK with you and you prefer to handle the standalone stuff yourself please tell me, I'll update the PR to keep the Libretro commit only ;)