There are two ways the first stroke point is moved after initial
placement:
- The `process_extension_sample` overwrites the initial values of the
sample point after `process_start_sample`. It copies the _new_ sample
position, until a threshold (3 pixels) is reached and the 2nd point
is created, and the 1st point remains stationary.
- After initial placement the point may still get shifted due to the
resampling and interpolation used. Long sections between stroke
samples may get subdivided and the positions of the two samples are
linearly interpolated. However, the interpolation starts at 1/n,
meaning the first interpolated point never matches the first sample.
This is correct for later samples where the last point should not be
repeated, but it ends up moving the first curve point again when the
2nd sample is processed.
This patch fixes both issues by keeping the first generated point
stationary and never touching its position again.
Pull Request: https://projects.blender.org/blender/blender/pulls/121011
11bfac7f11