Ensure Matplotlib pane handles explicit width/height settings correctly

[philippjfr]

Handle aspect ratio calculations correctly when explicit width/height is set.

[codecov]

Codecov Report

Merging #4128 (6ce5298) into master (b0fc124) will decrease coverage by 8.98%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #4128      +/-   ##
==========================================
- Coverage   83.52%   74.54%   -8.99%     
==========================================
  Files         224      224              
  Lines       33357    33373      +16     
==========================================
- Hits        27863    24878    -2985     
- Misses       5494     8495    +3001     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	74.54% <77.77%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
panel/pane/plot.py	61.84% <77.77%> (+0.99%)	⬆️
panel/tests/ui/widgets/test_tabulator.py	9.55% <0.00%> (-89.42%)	⬇️
panel/tests/ui/widgets/test_input.py	11.34% <0.00%> (-88.13%)	⬇️
panel/tests/ui/layout/test_accordion.py	14.72% <0.00%> (-85.28%)	⬇️
panel/tests/ui/widgets/test_sliders.py	15.42% <0.00%> (-84.58%)	⬇️
panel/tests/ui/layout/test_card.py	18.01% <0.00%> (-81.99%)	⬇️
panel/tests/ui/io/test_convert.py	15.92% <0.00%> (-77.00%)	⬇️
panel/tests/ui/test_reactive.py	26.53% <0.00%> (-73.47%)	⬇️
panel/tests/ui/template/test_fastgridtemplate.py	28.57% <0.00%> (-71.43%)	⬇️
panel/tests/ui/template/test_fastlisttemplate.py	28.57% <0.00%> (-71.43%)	⬇️
... and 32 more

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