Ensure only fractional floats are converted to SkFixed in SubpixelAlignment.

src/core/SkFindAndPlaceGlyph.h

Index: src/core/SkFindAndPlaceGlyph.h
diff --git a/src/core/SkFindAndPlaceGlyph.h b/src/core/SkFindAndPlaceGlyph.h
index 18b93f70a466291cc1f44989f3d75463442ad919..35ae77cfe3191c2cf8ff4dbfd54e1cb048bbfd8c 100644
--- a/src/core/SkFindAndPlaceGlyph.h
+++ b/src/core/SkFindAndPlaceGlyph.h
@@ -381,14 +381,16 @@ private:
     // produce the correct sub-pixel alignment. If a position is aligned with an axis a shortcut
     // of 0 is used for the sub-pixel position.
     static SkIPoint SubpixelAlignment(SkAxisAlignment axisAlignment, SkPoint position) {
+        // Only the fractional part of position.fX and position.fY matter, because the result of
+        // this function will just be passed to FixedToSub.
         switch (axisAlignment) {
             case kX_SkAxisAlignment:
-                return {SkScalarToFixed(position.fX + kSubpixelRounding), 0};
+                return {SkScalarToFixed(SkScalarFraction(position.fX) + kSubpixelRounding), 0};

[bungeman-skia, 2016/03/10 20:55:58]

SkScalarFraction is currently really super slow. I'll see about speeding it up a
bit first... https://codereview.chromium.org/1783583003/

[dogben, 2016/03/10 21:16:22]

I maybe ought to always return positive numbers too. Or just return
SubpixelRound units.

[bungeman-skia, 2016/03/10 21:37:49]

Always returning positive numbers would just be extra work. I suppose if we had
SkSubpixelRound units we could save a shift since (in the big picture) we really
just want the fractional part * 2^SkGlyph::kSubBits as an int so we can
discretize. (These values are just passed along until they are shoved into a
call to SkGlyph::MakeID.) But that's more difficult to do.

             case kY_SkAxisAlignment:
-                return {0, SkScalarToFixed(position.fY + kSubpixelRounding)};
+                return {0, SkScalarToFixed(SkScalarFraction(position.fY) + kSubpixelRounding)};
             case kNone_SkAxisAlignment:
-                return {SkScalarToFixed(position.fX + kSubpixelRounding),
-                        SkScalarToFixed(position.fY + kSubpixelRounding)};
+                return {SkScalarToFixed(SkScalarFraction(position.fX) + kSubpixelRounding),
+                        SkScalarToFixed(SkScalarFraction(position.fY) + kSubpixelRounding)};
         }
         SkFAIL("Should not get here.");
         return {0, 0};