fpdf: introduce buffer pool for (de)flating buffers

[sbinet]

name              old time/op    new time/op    delta
ParsePNG_rgb-8      22.8ms ± 1%    16.0ms ± 3%  -29.81%  (p=0.000 n=25+30)
ParsePNG_gray-8      660µs ± 3%     624µs ± 4%   -5.54%  (p=0.000 n=28+30)
ParsePNG_small-8    2.49µs ± 2%    2.12µs ± 3%  -14.83%  (p=0.000 n=30+30)
ParseJPG-8          36.3µs ± 4%    36.8µs ± 5%     ~     (p=0.070 n=30+30)
ParseGIF-8           410µs ± 3%     409µs ± 4%     ~     (p=0.562 n=30+29)

name              old alloc/op   new alloc/op   delta
ParsePNG_rgb-8      26.4MB ± 0%     4.2MB ±10%  -84.04%  (p=0.000 n=29+30)
ParsePNG_gray-8     2.56MB ± 0%    2.49MB ± 0%   -2.73%  (p=0.000 n=30+30)
ParsePNG_small-8    8.96kB ± 0%    9.84kB ± 0%   +9.73%  (p=0.000 n=30+30)
ParseJPG-8           274kB ± 0%     274kB ± 0%   -0.00%  (p=0.002 n=30+30)
ParseGIF-8           897kB ± 0%     898kB ± 0%   +0.10%  (p=0.000 n=30+30)

name              old allocs/op  new allocs/op  delta
ParsePNG_rgb-8         400 ± 0%       292 ± 0%  -27.00%  (p=0.000 n=30+28)
ParsePNG_gray-8        164 ± 0%       114 ± 0%  -30.49%  (p=0.000 n=30+30)
ParsePNG_small-8      35.0 ± 0%      10.0 ± 0%  -71.43%  (p=0.000 n=30+30)
ParseJPG-8            12.0 ± 0%      12.0 ± 0%     ~     (all equal)
ParseGIF-8             214 ± 0%       191 ± 0%  -10.75%  (p=0.000 n=30+30)

[codecov-commenter]

Codecov Report

Merging #8 (fc0b6fc) into main (0532bfd) will decrease coverage by 0.00%.
The diff coverage is 77.48%.

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   79.26%   79.25%   -0.01%     
==========================================
  Files          29       32       +3     
  Lines        5459     5524      +65     
==========================================
+ Hits         4327     4378      +51     
- Misses        817      827      +10     
- Partials      315      319       +4     

Impacted Files	Coverage Δ
util.go	72.82% <ø> (+0.18%)	⬆️
template.go	79.31% <20.00%> (-1.94%)	⬇️
fpdf.go	82.08% <60.00%> (ø)
xcompr.go	67.64% <67.64%> (ø)
rbuf.go	73.33% <73.33%> (ø)
png.go	78.66% <91.80%> (+3.28%)	⬆️
attachments.go	92.72% <100.00%> (+0.27%)	⬆️
wbuf.go	100.00% <100.00%> (ø)
... and 1 more

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