Fix multi-channel handling

[timtreis]

I think the current implementation doesn't correctly handle images with 3+ channels, for example resulting in the following warning when rendering a 5 channel image:

Clipping input data to the valid range for imshow with RGB data ([0..1] for floats or [0..255] for integers). Got range [0.01870049980776624..2.4537485582468284].

This PR proposes an alternative in which PCA is used to reduce an arbitrary amount of channels to 3 so that we can treat them as RGB:

[timtreis]

@Sonja-Stockhaus did we discuss this at one point? I've been ignoring that warning for a long time because it's thrown my matplotlib and doesn't fail

[timtreis]

This PR also introduces a bg_threshold parameter based on which we can exclude pixels for the PCA normalisation, f.e. in this composite

[Sonja-Stockhaus]

@Sonja-Stockhaus did we discuss this at one point? I've been ignoring that warning for a long time because it's thrown my matplotlib and doesn't fail

Nope. I also don't remember ever trying to render sth with 3+ channels

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.59794% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.71%. Comparing base (7716d43) to head (2e991c5).
⚠️ Report is 68 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spatialdata_plot/pl/render.py	88.05%	8 Missing ⚠️
src/spatialdata_plot/pl/utils.py	81.48%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #451      +/-   ##
==========================================
+ Coverage   84.92%   85.71%   +0.79%     
==========================================
  Files           8        8              
  Lines        1784     1869      +85     
==========================================
+ Hits         1515     1602      +87     
+ Misses        269      267       -2     

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/basic.py	89.26% <100.00%> (ø)
src/spatialdata_plot/pl/render_params.py	100.00% <100.00%> (ø)
src/spatialdata_plot/pl/utils.py	79.39% <81.48%> (+1.19%)	⬆️
src/spatialdata_plot/pl/render.py	95.52% <88.05%> (-0.86%)	⬇️

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