Add videosink display driver

[hansmi]

The videosink package provides a display driver implementing an HTTP
request handler. Client requests get an initial snapshot of the graphics
buffer and are updated further on every change.

Provides the basic implementation for #26. Still to be done are example code and periph/cmd integration.

[codecov-commenter]

Codecov Report

Merging #27 (031904a) into main (02831f4) will increase coverage by 0.9%.
The diff coverage is 87.7%.

@@           Coverage Diff           @@
##            main     #27     +/-   ##
=======================================
+ Coverage   60.3%   61.2%   +0.9%     
=======================================
  Files         48      53      +5     
  Lines       6453    6672    +219     
=======================================
+ Hits        3893    4085    +192     
- Misses      2425    2446     +21     
- Partials     135     141      +6     

Impacted Files	Coverage Δ
videosink/handler.go	82.5% <82.5%> (ø)
videosink/display.go	87.5% <87.5%> (ø)
videosink/writer.go	93.8% <93.8%> (ø)
videosink/encoder.go	100.0% <100.0%> (ø)
videosink/format.go	100.0% <100.0%> (ø)

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[maruel]

Thanks for the change! Nothing structural, just minutia questions.

[maruel]

I decided to merge, let's continue the discussion if we want to rename things a bit before next release. Even then it's not a big deal if we break it while there isn't users yet.