Test and fix SkPMFloat rounding.

tests/PMFloatTest.cpp

 #include "SkPMFloat.h"
 #include "Test.h"
 
 DEF_TEST(SkPMFloat, r) {
     // Test SkPMColor <-> SkPMFloat
     SkPMColor c = SkPreMultiplyColor(0xFFCC9933);
 
     SkPMFloat pmf;
     pmf.set(c);
     REPORTER_ASSERT(r, SkScalarNearlyEqual(255.0f, pmf.a()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual(204.0f, pmf.r()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual(153.0f, pmf.g()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual( 51.0f, pmf.b()));
+    REPORTER_ASSERT(r, c == pmf.get());
 
+    // Test rounding.  (Don't bother testing .5... we don't care which way it goes.)
+    pmf.setA(254.6f);
+    pmf.setR(204.3f);
+    pmf.setG(153.1f);
+    pmf.setB( 50.8f);
     REPORTER_ASSERT(r, c == pmf.get());
 
     // Test clamping.
     SkPMFloat unclamped;
     unclamped.setA(+510.0f);
     unclamped.setR(+153.0f);
     unclamped.setG(  +1.0f);
     unclamped.setB(  -0.2f);
 
     SkPMFloat clamped;
     clamped.set(unclamped.clamped());
 
     REPORTER_ASSERT(r, SkScalarNearlyEqual(255.0f, clamped.a()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual(153.0f, clamped.r()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual(  1.0f, clamped.g()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual(  0.0f, clamped.b()));
 
     // Test SkPMFloat <-> Sk4f conversion.
     Sk4f fs = clamped;
     SkPMFloat scaled = fs.multiply(0.25f);
 
     REPORTER_ASSERT(r, SkScalarNearlyEqual(63.75f, scaled.a()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual(38.25f, scaled.r()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual( 0.25f, scaled.g()));
     REPORTER_ASSERT(r, SkScalarNearlyEqual( 0.00f, scaled.b()));
 }