add legacy eval cubic

An upcoming CL in Skia uses SIMD instructions to
compute the coefficients for cubic and quadratic
polynomials. This introduces some minor differences.

Add a legacy flag to permit cubic eval() to work
as it did in the past until the appropriate
layout tests can be rebaselined.

R=fmalita@chromium.org

Review URL: https://codereview.chromium.org/1632313002

Cr-Commit-Position: refs/heads/master@{#371606}

skia/config/SkUserConfig.h

@@ -234,6 +234,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #   define SK_SUPPORT_LEGACY_BITMAP_FILTER
 #endif
 
+#ifndef    SK_SUPPORT_LEGACY_EVAL_CUBIC
+#   define SK_SUPPORT_LEGACY_EVAL_CUBIC
+#endif
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,