Fixing GetMaxFrame() to fix tiny float overshoot

[jonoomph]

Fixing GetMaxFrame() to fix tiny float overshoot, causing occasional black frames at the end of an export. For example, it might have concluded that we had 505.00001 frames, and this would then be converted later to 506 with ceil().

[jonoomph]

Exported project ends with a blank frame instead of the last frame of the imported video file.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.12%. Comparing base (fbc37e2) to head (6ccb526).

Files with missing lines	Patch %	Lines
src/Timeline.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1070      +/-   ##
===========================================
+ Coverage    70.11%   70.12%   +0.01%     
===========================================
  Files          241      241              
  Lines        29059    29075      +16     
===========================================
+ Hits         20375    20390      +15     
- Misses        8684     8685       +1     

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