skillcalc: use double precision for xp

Doing the xp calculation with floats and then converting it to double is
less precise, and causes minor errors that accumulate with very large xp
ranges

runelite-client/src/main/java/net/runelite/client/plugins/skillcalculator/SkillCalculator.java

@@ -343,9 +343,8 @@ class SkillCalculator extends JPanel
 			int actionCount = 0;
 			int neededXP = targetXP - currentXP;
 			SkillAction action = slot.getAction();
-			float xp = action.isIgnoreBonus()
-				? action.getXp()
-				: Math.round(action.getXp() * xpFactor * 10f) / 10f;
+			final float bonus = action.isIgnoreBonus() ? 1f : xpFactor;
+			final double xp = Math.round(action.getXp() * bonus * 10f) / 10d;
 
 			if (neededXP > 0)
 			{