Test and fix SkPMFloat rounding.

src/opts/SkPMFloat_neon.h

 #include "SkColorPriv.h"
 #include <arm_neon.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.
 // clamped() uses vqmovn to clamp while narrowing instead of just narrowing with vmovn.
 
 inline void SkPMFloat::set(SkPMColor c) {
     SkPMColorAssert(c);
     uint8x8_t   fix8    = (uint8x8_t)vdup_n_u32(c);
     uint16x8_t  fix8_16 = vmovl_u8(fix8);
     uint32x4_t  fix8_32 = vmovl_u16(vget_low_u16(fix8_16));
     vst1q_f32(fColor, vcvtq_f32_u32(fix8_32));
     SkASSERT(this->isValid());
 }
 
 inline SkPMColor SkPMFloat::get() const {
     SkASSERT(this->isValid());
-    uint32x4_t  fix8_32 = vcvtq_u32_f32(vld1q_f32(fColor));
-    uint16x4_t  fix8_16 = vmovn_u32(fix8_32);
-    uint8x8_t   fix8    = vmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
+    float32x4_t add_half = vaddq_f32(vld1q_f32(fColor), vdupq_n_f32(0.5f));
+    uint32x4_t  fix8_32  = vcvtq_u32_f32(add_half);  // vcvtq_u32_f32 truncates, so round manually
+    uint16x4_t  fix8_16  = vmovn_u32(fix8_32);
+    uint8x8_t   fix8     = vmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
     SkPMColor c = vget_lane_u32((uint32x2_t)fix8, 0);
     SkPMColorAssert(c);
     return c;
 }
 
 inline SkPMColor SkPMFloat::clamped() const {
-    uint32x4_t  fix8_32 = vcvtq_u32_f32(vld1q_f32(fColor));
-    uint16x4_t  fix8_16 = vqmovn_u32(fix8_32);
-    uint8x8_t   fix8    = vqmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
+    float32x4_t add_half = vaddq_f32(vld1q_f32(fColor), vdupq_n_f32(0.5f));
+    uint32x4_t  fix8_32  = vcvtq_u32_f32(add_half);  // vcvtq_u32_f32 truncates, so round manually
+    uint16x4_t  fix8_16  = vqmovn_u32(fix8_32);
+    uint8x8_t   fix8     = vqmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
     SkPMColor c = vget_lane_u32((uint32x2_t)fix8, 0);
     SkPMColorAssert(c);
     return c;
 }