Hudgauge firepoint scales maybe should be floats

`hudreticle.h`
```
class HudGaugeReticle: public HudGauge
{
protected:
```
...
```
SCP_vector fp;

int firepoint_size;
int firepoint_scale_x;
int firepoint_scale_y;
}
```

Brought up by 0rph3u5 in discord, apparently these scale values, which one would intuitively expect to be floats, are not floats. They're set by `+Center Reticle: Firepoint X coordinate multiplier:` and such in hud gauges. This came to light because the recent stricter parsing now errors on values like `6.0`.

I can see some argument for positions and sizes of hudgauges to be ints, though I'm not sure it's an argument I'd agree with, but scaling things by non-whole numbers seems like a pretty likely thing to try to do.

Then again, it's been like this for apparently 11 years. As it is now, old mods with custom firepoint scalings are going to probably error, but if it's adjusted engine side so those won't error, things might wiggle around a bit unexpectedly.

Thought it should be filed in any case.