add and test SkFixed15::to_u8()

Change-Id: Iedbcd2d938122cdc8f6b235745eb6165e348c237
Reviewed-on: https://skia-review.googlesource.com/7108
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>

src/core/SkFixed15.h

@@ -32,6 +32,12 @@ public:
              + ((val+1)>>8);       // All val but 255 are correct.  +1 if val == 255 to get 32768.
     }
 
+    uint8_t to_u8() const {
+        // FromU8() and to_u8() roundtrip all bytes.
+        // There is still much room to tweak this towards the ideal, a rounding scale by 255/32768.
+        return (fVal - (fVal>>8))>>7;
+    }
+
     SkFixed15 operator +(SkFixed15 o) const { return fVal + o.fVal; }
     SkFixed15 operator -(SkFixed15 o) const { return fVal - o.fVal; }
     SkFixed15 operator *(SkFixed15 o) const { return (fVal * o.fVal + (1<<14)) >> 15; }

tests/SkFixed15Test.cpp

@@ -26,4 +26,9 @@ DEF_TEST(SkFixed15, r) {
     for (int x = 0; x < 256; x++) {
         REPORTER_ASSERT(r, SkFixed15::FromU8(x) == SkFixed15(x * (1/255.0f)));
     }
+
+    // to_u8() and FromU8() should roundtrip all bytes.
+    for (int x = 0; x < 256; x++) {
+        REPORTER_ASSERT(r, x == SkFixed15::FromU8(x).to_u8());
+    }
 }