Nuke Extracts Thumbnail from frame out of shot range

[jrsndl]

Describe the bug
Nuke extracts thumbnail from middle frame. Currently it is calculated as last_frame / 2. This works nicely if shot range starts from 1. However, vfx often starts from 1001, and Nuke is extracted from frame 500. Depending on Nuke script, this might be OK (can still hold first frame) or produce pure garbage.

To Reproduce

1. Set shot range 1001-1010
2. Set plate read node before and after knobs to black (default is hold)
3. Locally render and publish version
4. The thumbnail is black (frame 500)

Expected behavior
Extract Thumbnail should make thumbnail in shot range, from middle frame.

Desktop (please complete the following information):

• OS: Windows 10
• Host: Nuke 10.3

Possible Fix
Calculating first_frame and last_frame with start frame offset:
https://github.com/pypeclub/pype/blob/5f48a59ce209d3c7629ecbb4c39860d2df3ff565/pype/plugins/nuke/publish/extract_thumbnail.py#L121

mid_frame = int((int(last_frame) - int(first_frame) ) / 2) + int(first_frame)
first_frame = int(last_frame) / 2
last_frame = int(last_frame) / 2

repre = {
    'name': name,
    'ext': "jpeg",
    "outputName": "thumb",
    'files': file,
    "stagingDir": staging_dir,
    "frameStart": first_frame,
    "frameEnd": last_frame,
    "tags": tags
}
instance.data["representations"].append(repre)

# Render frames
nuke.execute(write_node.name(), int(mid_frame), int(mid_frame))