gr: support :pixel

[t-bltg]

gr actually supports the :pixel marker (MARKERTYPE_DOT): https://gr-framework.org/markertypes.html.

[codecov]

Codecov Report

Merging #4283 (ca2c83f) into master (1cafdb0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4283   +/-   ##
=======================================
  Coverage   79.67%   79.67%           
=======================================
  Files          29       29           
  Lines        7052     7052           
=======================================
  Hits         5619     5619           
  Misses       1433     1433           

Impacted Files	Coverage Δ
src/backends.jl	64.60% <ø> (ø)
src/backends/gr.jl	88.57% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1cafdb0...ca2c83f. Read the comment docs.