Video: add screenshot function

[Emantor]

Description

Add screenshot support to the VideoProtocol and implement it for both drivers. Checklist

• [x] PR has been tested (USBVideo part)

[jluebbe]

This should probably be exposed in labgrid-client as well, perhaps similar to tmc screen? In that case, the drivers API provides the data in memory, so the user code can decide what to do with it.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 23.40426% with 36 lines in your changes missing coverage. Please review.

Project coverage is 55.6%. Comparing base (71fe895) to head (9ede8cc). Report is 117 commits behind head on master.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/usbvideodriver.py	11.1%	24 Missing :warning:
labgrid/driver/httpvideodriver.py	33.3%	12 Missing :warning:

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1612     +/-   ##
========================================
- Coverage    55.8%   55.6%   -0.2%     
========================================
  Files         170     170             
  Lines       13380   13427     +47     
========================================
+ Hits         7469    7478      +9     
- Misses       5911    5949     +38     

Flag	Coverage Δ
3.10	55.9% <23.4%> (-0.2%)	:arrow_down:
3.11	55.9% <23.4%> (-0.2%)	:arrow_down:
3.12	55.9% <23.4%> (-0.2%)	:arrow_down:
3.13	55.9% <23.4%> (-0.2%)	:arrow_down:
3.9	56.0% <23.9%> (-0.2%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.