As per lastest revision of the spec, we can specify amounts in invoice
requests even if the offer already specifies it, as long as we exceed
the amount given. This allows for tipping, and amount obfuscation.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Means a reshuffle of our logic: we want to multiply by quantity before
conversion for maximum accuracy.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>