STY: Refactor _xobj_to_image

[MartinThoma]

• Create _apply_decode and move it to _xobj_image_helpers
• Create _get_mode_and_invert_color and move it to _xobj_image_helpers
• Create _apply_alpha as an inner function as it's calling _xobj_to_image

This reduced the Cyclomatic Complexity from 44 to 19

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.00%. Comparing base (966e015) to head (420f3b1). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pypdf/filters.py	91.30%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2863 +/- ## ======================================= Coverage 96.00% 96.00% ======================================= Files 51 51 Lines 8528 8539 +11 Branches 1699 1700 +1 ======================================= + Hits 8187 8198 +11 Misses 200 200 Partials 141 141 ```

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