Fix annotations getting drawn under markers in pgfplotsx backend

[chwons]

When adding annotations to a scatter plot, the annotations get drawn under the markers.
I added the option clip mode = individual to fix this.

[codecov]

Codecov Report

Merging #3957 (018783b) into master (c94cf18) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3957      +/-   ##
==========================================
+ Coverage   62.45%   62.49%   +0.04%     
==========================================
  Files          29       29              
  Lines        7223     7223              
==========================================
+ Hits         4511     4514       +3     
+ Misses       2712     2709       -3     

Impacted Files	Coverage Δ
src/backends/pgfplotsx.jl	60.18% <ø> (ø)
src/args.jl	71.71% <0.00%> (-0.31%)	⬇️
src/backends/gr.jl	88.40% <0.00%> (+0.41%)	⬆️

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[BeastyBlacksmith]

Thanks!