fix: fix composite mode on init

[tlambert03]

fixes #197

The core problem was that the whole "guess channel axis" hadn't triggered by the time the canvas was drawn. the example works fine if you pass in an explicit channel axis, so we just need to do a better job of coordinating the timing of that magic.

This gets the example working... but I need to check whether some other things have been broken

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.07%. Comparing base (e3e94d3) to head (3cdfa92).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ndv/models/_data_display_model.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #200      +/-   ##
==========================================
+ Coverage   85.92%   86.07%   +0.14%     
==========================================
  Files          46       46              
  Lines        5202     5200       -2     
==========================================
+ Hits         4470     4476       +6     
+ Misses        732      724       -8     

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