Added support for user defined sound effect implementations.

[binary1248]

Title.

Because I found it a pity that #1708 had to be closed due to the miniaudio rewrite and all of @fallahn's work gone to waste, I went ahead and implemented equivalent (maybe even better?) support for user defined sound effects in the spirit of aforementioned PR.

The example has been accordingly extended with demonstrations of how to use the new API to implement basically any effect one can think of. I am by no means a DSP expert (the example implementations were taken from people far more knowledgeable than me 😁), so maybe someone can improve the example processing code or even add more interesting effects.

All of the action happens when an effect processor is attached to an sf::SoundSource using:

using EffectProcessor = std::function<void(const float*, unsigned int&, float*, unsigned int&, unsigned int)>;

void setEffectProcessor(EffectProcessor effectProcessor);

The effect processor is passed the raw audio data from the engine and can manipulate it before sending it back to be output through the audio device. Refer to the sf::SoundSource documentation and sound effects example for detailed information on API usage.

[ChrisThrasher]

Is there anything we can do to reduce the code duplication between Sound.cpp and SoundStream.cpp?

[coveralls]

Pull Request Test Coverage Report for Build 9122177114

Details

• 80 of 130 (61.54%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 55.481%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/SFML/Audio/Sound.cpp	40	65	61.54%
src/SFML/Audio/SoundStream.cpp	40	65	61.54%

Totals
Change from base Build 9119260362:	0.02%
Covered Lines:	11502
Relevant Lines:	19622

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💛 - Coveralls

[binary1248]

Is there anything we can do to reduce the code duplication between Sound.cpp and SoundStream.cpp?

Not really... most of the duplicated code references internal members of Impl and since Impl is local to each TU trying to refactor out those blocks would just make the code unnecessarily more complex.

[binary1248]

Rebased onto master.

[vittorioromeo]

Mostly nitpicks

[binary1248]

Updated with @vittorioromeo's suggestions.

[ChrisThrasher]

Rebased on master to inherit the new Audio module tests