use premul_alpha everywhere

[MyreMylar]

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[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 96.28%. Comparing base (c125c30) to head (c163217).
Report is 9 commits behind head on main.

Files	Patch %	Lines
pygame_gui/core/ui_appearance_theme.py	75.00%	2 Missing ⚠️
pygame_gui/elements/ui_text_entry_line.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #597      +/-   ##
==========================================
+ Coverage   96.22%   96.28%   +0.05%     
==========================================
  Files          89       89              
  Lines       13342    13558     +216     
==========================================
+ Hits        12839    13054     +215     
- Misses        503      504       +1     

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