Simplify affine matrix calculation for datashader

[LucaMarconato]

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I simplified the code from https://github.com/scverse/spatialdata-plot/pull/378 (taking https://github.com/scverse/spatialdata-plot/issues/381 into account), now the test pass using a simpler construction of the matrix.

CC @Sonja-Stockhaus

[codecov-commenter]

Codecov Report

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

Project coverage is 83.63%. Comparing base (478ab27) to head (55a1c14).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #424      +/-   ##
==========================================
- Coverage   83.83%   83.63%   -0.20%     
==========================================
  Files           8        8              
  Lines        1732     1711      -21     
==========================================
- Hits         1452     1431      -21     
  Misses        280      280              

Files with missing lines	Coverage Δ
src/spatialdata_plot/pl/render.py	91.14% <100.00%> (-0.03%)	:arrow_down:
src/spatialdata_plot/pl/utils.py	77.56% <100.00%> (-0.45%)	:arrow_down: