Fix SLSTR composites for oblique view

[mraspaud]

This makes the view id key transitive for composites too.

• Closes Loading SLSTR composite doesn't respect the view #1532

[codecov]

Codecov Report

Merging #1533 (982bb06) into master (e0f1fb8) will decrease coverage by 1.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1533      +/-   ##
==========================================
- Coverage   91.65%   90.51%   -1.15%     
==========================================
  Files         248      248              
  Lines       36172    36172              
==========================================
- Hits        33155    32740     -415     
- Misses       3017     3432     +415     

Flag	Coverage Δ
behaviourtests	4.35% <ø> (ø)
unittests	91.03% <ø> (-1.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_seviri_l2_grib.py	23.07% <0.00%> (-76.93%)	⬇️
satpy/tests/reader_tests/test_seviri_l2_bufr.py	30.35% <0.00%> (-69.65%)	⬇️
satpy/readers/iasi_l2_so2_bufr.py	25.80% <0.00%> (-68.82%)	⬇️
satpy/readers/ascat_l2_soilmoisture_bufr.py	29.87% <0.00%> (-67.54%)	⬇️
satpy/tests/reader_tests/test_iasi_l2_so2_bufr.py	30.98% <0.00%> (-66.20%)	⬇️
...ts/reader_tests/test_ascat_l2_soilmoisture_bufr.py	37.20% <0.00%> (-60.47%)	⬇️
satpy/readers/seviri_l2_grib.py	26.53% <0.00%> (-60.21%)	⬇️
satpy/readers/seviri_l2_bufr.py	34.52% <0.00%> (-59.53%)	⬇️
satpy/writers/cf_writer.py	93.57% <0.00%> (-0.31%)	⬇️
satpy/scene.py	92.07% <0.00%> (-0.18%)	⬇️

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