Add test topic and exception for register_img_same

[ThePotatoGuy]

Copied from @multimokia:

Figured the issue out. It's an issue with init order. We should never use register_img_same on the same init level as register_img for the original, because it will always fail provided the tag does not exist yet.

Key Changes

• Adds a new exception in MASImageTagDecoDefinition.register_img_same when it could not find a deco definition from the source BG (we'll just assume that this is only being called at init time).
• Adds a testing topic DECO TAG TEST API REG SAME for checking the register_img_same function.

Testing

• in dev/dev_deco.rpy, uncomment the section under register_img_same init exception - verify that the game raises the new exception on load
• run the DECO TAG TEST API REG SAME dev topic - verify that the blue deco remains in the same place for each background.

[multimokia]

Figured the issue out. It's an issue with init order. We should never use register_img_same on the same init level as register_img for the original, because it will always fail provided the tag does not exist yet.

As such:
It might be worth just raising an error or logging in place of silently handling the issue as to actually provide some feedback to the user. Requesting some changes to add logging or an ex throw for it if config.developer

[ThePotatoGuy]

@multimokia I added an exception