Use geometric mean when selecting a mipmap scale

src/core/SkMipMap.cpp

Index: src/core/SkMipMap.cpp
diff --git a/src/core/SkMipMap.cpp b/src/core/SkMipMap.cpp
index 7266dc6b21192c16019c0277ea4387244033d209..c1751f01abc69a1b24fb86ee46db00f1f3524908 100644
--- a/src/core/SkMipMap.cpp
+++ b/src/core/SkMipMap.cpp
@@ -326,8 +326,16 @@ bool SkMipMap::extractLevel(const SkSize& scaleSize, Level* levelPtr) const {
     }
 
     SkASSERT(scaleSize.width() >= 0 && scaleSize.height() >= 0);
+
+#ifdef SK_SUPPORT_LEGACY_ANISOTROPIC_MIPMAP_SCALE

[reed1, 2016/02/16 21:37:56]

Written this way, we are going to change skia's default to be sqrt, different
from ganesh. I had thought we were adding this as an option for chrome only,
which they would opt into... Did you mean to change skia across the board?

[f(malita), 2016/02/16 21:46:23]

Indeed - I did not consider applying this to Chrome only.  As a workaround, that
makes sense.  Done.

     // Use the smallest scale to match the GPU impl.
     const SkScalar scale = SkTMin(scaleSize.width(), scaleSize.height());
+#else
+    // Ideally we'd pick the smaller scale, to match Ganesh.  But ignoring one of the
+    // scales may produce some attrocious results, so for now we use the geometric mean.
+    // (https://bugs.chromium.org/p/skia/issues/detail?id=4863)
+    const SkScalar scale = SkScalarSqrt(scaleSize.width() * scaleSize.height());
+#endif
 
     if (scale >= SK_Scalar1 || scale <= 0 || !SkScalarIsFinite(scale)) {
         return false;