fix: Allow PNGs to be displayed notebooks

[desmondcheongzx]

Changes Made

When we generate thumbnails to display in notebooks, we encoded them as JPEG by default. This does not work if images have an alpha channel.

This PR changes it so that we generate PNGs instead. This should have minimal performance impact because we're generating small 128x128 thumbnails and this only happens when previewing a small amount (typically <= 8) of results in a notebook.

Related Issues

Closes #4072

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.49%. Comparing base (d8535df) to head (40174a4). Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/daft-image/src/ops.rs	0.00%	2 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4255   +/-   ##
=======================================
  Coverage   78.48%   78.49%           
=======================================
  Files         798      798           
  Lines      105351   105375   +24     
=======================================
+ Hits        82687    82712   +25     
+ Misses      22664    22663    -1     

Files with missing lines	Coverage Δ
src/daft-image/src/ops.rs	85.71% <0.00%> (ø)

... and 10 files with indirect coverage changes

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