Fix #144 -- Improve DRM serialization performance by filtering source and ratio

[amureki]

This is an early draft of a possible direction.
I did not go all fancy and did not add tests before I validate this with you.

The manual test on 5000 objects showed me this:

DRF ImageField serialization TIME:  0.15182900428771973
PictureField serialization TIME:  12.384335041046143
PictureField serialization IMPROVED TIME:  3.4643609523773193

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (33562b8) 100.00% compared to head (7c7e943) 100.00%.
Report is 1 commits behind head on main.

Additional details and impacted files

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  Files           13        13           
  Lines          459       470   +11     
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+ Hits           459       470   +11     

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[codingjoe]

Excellent! This looks promising, I left you several fussy comments. But overall, once you add test, for the new kwargs, this is ready to ship.

[amureki]

@codingjoe got me to the question - should we handle cases when user only provides aspect ratio OR image source?
This would be a matter of a couple more elifs, or we also could go with some fancy matcher to solve that.

[codingjoe]

LGTM, thanks @amureki