1D Blend Linear Option

Feature request based on the issue I had here:

https://rive.app/community/forums/support/fsnLLkXUaA62/blend-1d-not-even/ftwB8mqTesbK

Basically, the 1D blend is not a linear blend. The example blend in the documentation shows a health bar which is misleading seeing how 50% is more like 70% if the intention is to reflect an actual value. The thread outlines some workarounds including a reset layer before the 1D Blend and and Additive Blend. The reset blend is not a great solution for a couple of reasons.

1. In my case, I wanted to animate a health drain animation so I only run the blend on a trigger with a transition time. The reset layer negates the animation.

2. The best practices doc says "When blend states are activated at runtime, Rive will continually play the state machine, even if it is not necessary anymore" so having a reset layer requires the blend to constantly run.

For my purposes, using an additive blend was the best solution, but having a simple checkbox to specify a 1D blend as linear would help in a lot of situations.