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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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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[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.