Make it possible to change the curve of 1D blend states (and fix the bug)

Also, fix the bug where blend states aren't linear by default and you have to put a layer before the blend layer that defaults to the first blend timeline. I understand this may not be a bug by technicality for some mathematical reason, but it's still a bug because it defies user expectations about how the product should work, and it's frustrating to everyone who tries to use 1D blend states. Bugs don't always exist for technical reasons.

TLDR: make 1D blend states linear by default and allow users to alter the curve like they can with keyframes by dragging the curves around: