avm1: target_clip behavior difference depending on SWF version

[relrelb]

On SWF6, calling a function changes the active target_clip according to the callee's definition location.
Currently Ruffle handles this behavior as expected:

ruffle/core/src/avm1/function.rs

Line 324 in 9df7fac

af.base_clip,

However SWF5 seems to behave differently. Instead of changing the active target_clip, it remains unchanged and same as the caller's target_clip.
Ruffle doesn't handle this behavior and always behaves like SWF6.

The attached examples are identical except for their SWF version (5 vs. 6).
Notice that only the yellow circle moves in target_clip_swf5.zip, but both the yellow circle and the green square move in target_clip_swf6.zip.

target_clip_swf5.zip
target_clip_swf6.zip

Changing the line mentioned above from af.base_clip, to activation.base_clip, (plus making it pub) seem to change the behavior from SWF6 to SWF5. I feel like selecting either af.base_clip or activation.base_clip depending on the SWF version is the proper fix, but I'm not 100% sure about it.

[Herschel]

Seems accurate to me, would you like to make a PR with the fix and test case? Here's another test case using gotoAndPlay (which affects the target clip):
gotoandplay_swf5.zip