Applies the same fixes to multiply blending in the AVX2
optimized path that were previously applied to the SSE2 path.
Namely, alpha blending and colors now both blend correctly
The way the SIMD blitting paths were previously organized
ensured that the AVX2 one would never actually build. This
commit separates the different implementations into
classes with differing names, ensuring the shadowing
cannot occur.
The blit-neon.cpp file is only added to the project if the flag
SCUMMVM_NEON is defined. However some backends, e.g. iOS, adds the
definition of SCUMMVM_NEON in create_project so the file can be
added to the project during configuration.
The same backend can then unset the definition due to changing
build target, the iOS simulator in this example, which runs can
run on x86_64 hosts which doesn't support NEON.
Since the project still include the source file it will be compiled.
Make sure to not build the file if not SCUMMVM_NEON is defined at
compile time.
