Test and fix SkPMFloat rounding.

src/opts/SkPMFloat_SSE2.h

 #include "SkColorPriv.h"
 #include <emmintrin.h>
 
 // For set(), we widen our 8 bit components (fix8) to 8-bit components in 16 bits (fix8_16),
 // then widen those to 8-bit-in-32-bits (fix8_32), and finally convert those to floats.
 
 // get() and clamped() do the opposite, working from floats to 8-bit-in-32-bit,
 // to 8-bit-in-16-bit, back down to 8-bit components.
 // _mm_packus_epi16() gives us clamping for free while narrowing.
 
 inline void SkPMFloat::set(SkPMColor c) {
     SkPMColorAssert(c);
     __m128i fix8    = _mm_set_epi32(0,0,0,c),
             fix8_16 = _mm_unpacklo_epi8 (fix8,    _mm_setzero_si128()),
             fix8_32 = _mm_unpacklo_epi16(fix8_16, _mm_setzero_si128());
     _mm_store_ps(fColor, _mm_cvtepi32_ps(fix8_32));
     SkASSERT(this->isValid());
 }
 
 inline SkPMColor SkPMFloat::get() const {
     SkASSERT(this->isValid());
     return this->clamped();  // At the moment, we don't know anything faster.
 }
 
 inline SkPMColor SkPMFloat::clamped() const {
-    __m128i fix8_32 = _mm_cvtps_epi32(_mm_load_ps(fColor)),
+    __m128i fix8_32 = _mm_cvtps_epi32(_mm_load_ps(fColor)),  // _mm_cvtps_epi32 rounds for us!
             fix8_16 = _mm_packus_epi16(fix8_32, fix8_32),
             fix8    = _mm_packus_epi16(fix8_16, fix8_16);
     SkPMColor c = _mm_cvtsi128_si32(fix8);
     SkPMColorAssert(c);
     return c;
 }