Limit PAGDecoder scale to 1.0 for pure bitmap sequence compositions to avoid unnecessary upscaling.

[fengdyli]

为纯位图序列的 PAGComposition 添加缩放限制，避免无意义的放大操作，节省内存和性能。

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.28%. Comparing base (d836825) to head (6411cb0).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rendering/PAGDecoder.cpp	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3270      +/-   ##
==========================================
+ Coverage   77.04%   77.28%   +0.23%     
==========================================
  Files         413      415       +2     
  Lines       21922    22051     +129     
  Branches     6298     6312      +14     
==========================================
+ Hits        16890    17042     +152     
+ Misses       3821     3812       -9     
+ Partials     1211     1197      -14     

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