Do PNGFiles load check/import sequentially

[IanButterworth]

Loading via FileIO/ImageIO in threaded loops doesn't appear to be threadsafe https://discourse.julialang.org/t/segfault-while-loading-images-in-multiple-threads/48878/10

This ensures that the checking and loading of PNGFiles is done sequentially, the cost of which is negligible

The execution time of the new checked_import

0.530126 seconds (893.98 k allocations: 64.787 MiB, 2.17% gc time, 18.76% compilation time)
  0.000001 seconds
  0.000001 seconds
  0.000001 seconds
  0.000001 seconds
  0.000001 seconds

The matching PR in FileIO: JuliaIO/FileIO.jl#276

[codecov]

Codecov Report

Merging #11 into master will increase coverage by 19.04%.
The diff coverage is 68.75%.

@@             Coverage Diff             @@
##           master      #11       +/-   ##
===========================================
+ Coverage   47.61%   66.66%   +19.04%     
===========================================
  Files           1        1               
  Lines          21       21               
===========================================
+ Hits           10       14        +4     
+ Misses         11        7        -4     

Impacted Files	Coverage Δ
src/ImageIO.jl	66.66% <68.75%> (+19.04%)	⬆️

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