Blender Git Loki

Fix T70891: Motion Path - Changing endframe clamps startframe to 1

There were two strange things going on:

- Start frame was clamped to 1, even when frame 0 is always a valid number.
This also ignored the 'Allow Negative Frames' user preference.
- Start frame was only clamped when setting the end frame, so first setting
the end frame and then the start frame would result in a different result
than doing it in the opposite order.

This commit fixes both issues by:

- Clamping the lower bound of the start frame only if negative frames are
not allowed, and
- apply that clamp both when setting the start and the end frame.