NLA: Remove Hold resetting between Hold_Forward Behavior

Before, moving (or Push-Down Action-ing) a Replace strip with extrap=`Hold or Hold_Forward` will auto reset to the other based on whichever first occurs among all the tracks. Now, that auto reset behavior has been completely removed and //only// that behavior.

This patch does not change strip evaluation. Between two strips, the first with `None` extrapolation and the next with `Hold`, neither strip will evaluate, which matches previous behavior. A future patch can change the evaluation behavior.

Part of {T82230}

Differential Revision: https://developer.blender.org/D9942

NLA: Strip Evaluate Held Strips Even When Not First Strip

Changes NLA strip evaluation.

A visual example of the change (with current frame between both strips):

**Before**:
| First strip extrapolation | Next strip extrapolation | Evaluated Strip
| None | Hold | Neither
| Hold or Hold_Forward| Hold | First

**Now**:
| First strip extrapolation | Next strip extrapolation | Evaluated Strip
| None | Hold | Next
| Hold or Hold_Forward | Hold | First

There isn't a concrete use-case for this new behavior. It's more to avoid the case of [None | Hold | Neither] which appears buggy.

UI-wise, the case of `Hold`/`Hold_Forward` leading to a `Hold` will draw with a slightly darker extrapolation area overlay. Pre-extrapolation and post-extrapolation alpha is changed to the same value now too.

Part of {T82230}
This patch functionally depends on {D9942} to sanely test this patch.

Differential Revision: https://developer.blender.org/D9943