Add a skip for (re)calculating retargeting data on unchanged skeletons and mappings

[smix8]

Required retargeting data is currently always calculated when start_retargeting() is used (or editor button pressed).

While not very noticeable in the C++ module this can add a lot of additional processing time on the slow GDScript version.

After the first retargeting the data does not really need to be recalculated as long as:

• both skeletons (paths) stay the same
• mappings are not changed
• no user code from outside changes skeleton rest poses

A skip for (re)calculating offsets after the first retargeting and only reset when something changes could improve performance, especially for GDScript.

[smix8]

Added by commit 460b1bb0ad9715724258438f09d2869f7a4a4402 for Godot_3.x and commit 8969555c780c33900ba095a4bcc5e8bf905577e7 for Godot_3.x_gdscript.

Godot 4.x updates will come later, some additional quirks to solve.