Previously, the stroke operations were created in the `stroke_test_start`
callback function. This causes a few issues:
* Each mode had it's own `test_start` function and it's own function
to create the stroke operations.
* Only the mouse position was available for the start sample.
With this change, the `stroke_test_start` function now does nothing.
Instead, the operations are created in the common `stroke_update_step`
function using `get_stroke_operation` the first time it's called.
This also means that the start sample is populated with the correct
start pressure.
Pull Request: https://projects.blender.org/blender/blender/pulls/122555
1334d64780