improve heatmap_edges performance

[t-bltg]

Partial improvement for #4520.

using BenchmarkTools, Plots

const mat = randn(1024, 1024);

@btime show(devnull, heatmap($mat))
  35.233 ms (632 allocations: 8.10 MiB)  # PR
  35.512 ms (4670 allocations: 8.25 MiB)  # master

f(x, n = 100) = begin
    plot()
    for _ in 1:n
        heatmap!(x)
    end
end

@btime f($mat)
  2.320 s (42563 allocations: 807.30 MiB)  # PR
  2.348 s (446363 allocations: 822.83 MiB)  # master

[codecov]

Codecov Report

Base: 91.00% // Head: 90.83% // Decreases project coverage by -0.16% ⚠️

Coverage data is based on head (f702fb9) compared to base (04fefce).
Patch coverage: 89.23% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4539      +/-   ##
==========================================
- Coverage   91.00%   90.83%   -0.17%     
==========================================
  Files          40       40              
  Lines        7702     7694       -8     
==========================================
- Hits         7009     6989      -20     
- Misses        693      705      +12     

Impacted Files	Coverage Δ
src/utils.jl	92.52% <87.03%> (-0.95%)	⬇️
src/axes.jl	90.52% <100.00%> (-0.03%)	⬇️
RecipesPipeline/src/group.jl	90.54% <0.00%> (-4.06%)	⬇️
src/backends/gr.jl	91.63% <0.00%> (-0.36%)	⬇️
src/layouts.jl	95.44% <0.00%> (-0.29%)	⬇️
src/components.jl	89.81% <0.00%> (-0.27%)	⬇️
src/recipes.jl	84.55% <0.00%> (+0.23%)	⬆️

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