Added optimized version for alpha darken composite op for RGBF32 colorspace.
Added tests to test performance and results of new implementation against legacy.

The diff needed in the test compare is do to the fact that the compiler
calculates 1.0/255.0 and multiplying the result instead if dividing by
255.0 for the mask.

Here are the results of the benchmark on my Intel i5-2520M CPU

QDEBUG : KisCompositionBenchmark::testRgbF32CompositeAlphaDarkenLegacy() krita.general: Testing Composite Op: "alphadarken" ( "Legacy" )
QDEBUG : KisCompositionBenchmark::testRgbF32CompositeAlphaDarkenLegacy() krita.general: "Aligned   Mask   SrcRand DstRand" RESULT: 67 msec
QDEBUG : KisCompositionBenchmark::testRgbF32CompositeAlphaDarkenLegacy() krita.general: "DstUnalig Mask   SrcRand DstRand" RESULT: 68 msec
QDEBUG : KisCompositionBenchmark::testRgbF32CompositeAlphaDarkenLegacy() krita.general: "SrcUnalig Mask   SrcRand DstRand" RESULT: 69 msec
QDEBUG : KisCompositionBench...