excess over mintFeeAmount not returned

[dckc]

I'm doing some testing and I didn't have the mintFeeAmount handy, so I tried 50IST. Minting worked, but it took all 50IST. Some experimentation suggests the mintFeeAmount is 5IST, so I would expect to get 45IST back.

But the contract computes the platform / royalty split on the whole give.Price, not just mintFeeAmount:

KREAd/agoric/contract/src/kreadKit.js

Lines 331 to 332 in 56f72c4

	const royaltyFee = multiplyBy(give.Price, mintRoyaltyRate);
	const platformFee = multiplyBy(give.Price, mintPlatformFeeRate);