There were multiple places in the GPv3 code that assumed that the
frame key is equivalent to the start frame of the frame with that key.
But this is not the case. The `FramesMapKeyT` is either the start frame
*or* the end frame (for frames with fixed duration).
This adds a new function `start_frame_at` that returns the start frame
number of the frame at `frame_number` or -1 if no such frame exists.
One place needed the index into sorted keys (for onion skinning) so
this was replaced with a new function `sorted_keys_index_at`.
With these changes, `Layer::frame_key_at` is now a private method.
Pull Request: https://projects.blender.org/blender/blender/pulls/122045
125617dc82