Invalid invocation of vectorized copy2WithGain

[Uwe Klotz]

GCC 8.1.1 reveals aliasing of pointers that are passed as function arguments:

```
src/engine/enginemaster.cpp: In member function ‘void EngineMaster::process(int)’:

src/engine/enginemaster.cpp:554:43: warning: passing argument 1 to restrict-qualified parameter aliases with argument 2 [-Wrestrict]
                 SampleUtil::copy2WithGain(m_pMaster,
                                           ^~~~~~~~~
                     m_pMaster, 1.0,
                     ~~~~~~~~~
src/engine/enginemaster.cpp:585:43: warning: passing argument 1 to restrict-qualified parameter aliases with argument 2 [-Wrestrict]
                 SampleUtil::copy2WithGain(m_pMaster,
                                           ^~~~~~~~~
                     m_pMaster, 1.0,
                     ~~~~~~~~~
src/engine/enginemaster.cpp:664:43: warning: passing argument 1 to restrict-qualified parameter aliases with argument 2 [-Wrestrict]
                 SampleUtil::copy2WithGain(m_pSidechainMix,
                                           ^~~~~~~~~~~~~~~
                                           m_pSidechainMix, 1.0,
                                           ~~~~~~~~~~~~~~~
[CXX] src/engine/loopingcontrol.cpp
```

[RJ Skerry-Ryan (rryan)]

Oh no.. that's a disaster waiting to happen.

[RJ Skerry-Ryan (rryan)]

https://github.com/mixxxdj/mixxx/pull/1807

[RJ Skerry-Ryan (rryan)]

Will cherry-pick to 2.2 once merged.

[Swiftb0y (swiftb0y)]

Mixxx now uses GitHub for bug tracking. This bug has been migrated to:
https://github.com/mixxxdj/mixxx/issues/9285