Remove output parameter from sf::Image::saveToMemory

[ChrisThrasher]

Description

This is a textbook use case for a std::optional return value. This operation might return a buffer or it might not. It depends on whether the inputs are valid.

[codecov]

Codecov Report

Merging #2756 (6770e43) into master (0fcd1dd) will increase coverage by 0.01%.
The diff coverage is 84.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2756      +/-   ##
==========================================
+ Coverage   38.46%   38.47%   +0.01%     
==========================================
  Files         237      237              
  Lines       20799    20801       +2     
  Branches     5148     5152       +4     
==========================================
+ Hits         8000     8003       +3     
+ Misses      11843    11828      -15     
- Partials      956      970      +14     

Files	Coverage Δ
include/SFML/Graphics/Image.hpp	100.00% <ø> (ø)
src/SFML/Graphics/Image.cpp	90.47% <100.00%> (ø)
src/SFML/Graphics/ImageLoader.cpp	47.05% <81.81%> (+0.34%)	⬆️

... and 7 files with indirect coverage changes

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[kimci86]

It is now impossible to reuse or reserve vector memory upfront to save some memory allocations. No big deal I guess but I just wanted to point it out.