lottiemodel: Improve opacity calculation for gradient stop

Basically, Graeidnt stop's color and opacity are provided as separate arrays.
Stop position and opacity position do not match each other.
Existing code is a sequential approach. It caused problems in various cases of positions.
The improved logic repeats the loop, but no exceptions are raised. It's not complicated, it's simple.

This code referenced the lottie-android library.
https://github.com/airbnb/lottie-android/blob/master/lottie/src/main/java/com/airbnb/lottie/parser/GradientColorParser.java