Convert SkPMFloat to [0,1] range and prune its API.

src/opts/SkPMFloat_sse.h

+/*
+ * Copyright 2015 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+namespace {  // See SkPMFloat.h
+
+inline SkPMFloat::SkPMFloat(SkPMColor c) {
+    SkPMColorAssert(c);
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSSE3
+    const int _ = 255;  // Zero these bytes.
+    __m128i fix8    = _mm_cvtsi32_si128((int)c),
+            fix8_32 = _mm_shuffle_epi8(fix8, _mm_setr_epi8(0,_,_,_, 1,_,_,_, 2,_,_,_, 3,_,_,_));
+#else
+    __m128i fix8    = _mm_cvtsi32_si128((int)c),
+            fix8_16 = _mm_unpacklo_epi8 (fix8,    _mm_setzero_si128()),
+            fix8_32 = _mm_unpacklo_epi16(fix8_16, _mm_setzero_si128());
+#endif
+    fVec = _mm_mul_ps(_mm_cvtepi32_ps(fix8_32), _mm_set1_ps(1.0f / 255));
+    SkASSERT(this->isValid());
+}
+
+inline SkPMColor SkPMFloat::round() const {
+    // We don't use _mm_cvtps_epi32, because we want precise control over how 0.5 rounds (up).
+    __m128 scaled = _mm_mul_ps(_mm_set1_ps(255), fVec);
+    __m128i fix8_32 = _mm_cvttps_epi32(_mm_add_ps(_mm_set1_ps(0.5f), scaled)),
+            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;
+}
+
+}  // namespace