Fix resample reduce_data bug introduced in #582

[djhoese]

On slack @adybbroe pointed out that resampling was broken in current master. This was caused by some changes in #582 by @pnuu as part of refactoring the code. Bottom line, the source_area variable needs to represent the "reduced" area after reducing has been done.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage increased (+0.06%) to 78.135% when pulling d467662 on djhoese:bugfix-reduce-data into ea26c9b on pytroll:master.

[codecov]

Codecov Report

Merging #600 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #600      +/-   ##
==========================================
+ Coverage   78.07%   78.13%   +0.06%     
==========================================
  Files         136      136              
  Lines       19531    19552      +21     
==========================================
+ Hits        15248    15277      +29     
+ Misses       4283     4275       -8

Impacted Files	Coverage Δ
satpy/scene.py	88% <100%> (+1.33%)	⬆️
satpy/tests/test_scene.py	99.45% <100%> (+0.01%)	⬆️
satpy/resample.py	88.95% <0%> (+0.28%)	⬆️

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