Rainbow HUD: Fix invisible text being visible

[tmp64]

int CHud::GetHudStringWidth(const char* string)
{
    return gEngfuncs.pfnDrawString(0, 0, string, 0, 0, 0);
}

GetHudStringWidth gets the string length by drawing it with a black color (due to additive blending mode, it becomes transparent). Rainbow HUD overrides this color, causing the text to become visible.

This PR also fixes a small issue in DrawStringReverse. It used to call DrawString to get the string width. DrawString needs to decode the input string into separate UTF-8 chars to draw them with different colors but it isn't needed in this case. I replaced it with a call to CHud::GetHudStringWidth.

[YaLTeR]

Thanks