Use context start correctly in getRunAdvance

We were not taking context start into account when deciding whether
to split a ligature, which was causing inconsistent behavior. This
patch consistently references the widths array relative to the start
of the context.

Bug: 21549197
Change-Id: I7c71e10c1af84354fefe782fc0b87120016e6555

engine/src/flutter/libs/minikin/Measurement.cpp

@@ -41,7 +41,7 @@ float getRunAdvance(Layout& layout, const uint16_t* buf, size_t start, size_t co
             clusterWidth = charAdvance;
         }
     }
-    if (offset < start + count && layout.getCharAdvance(offset) == 0.0f) {
+    if (offset < start + count && layout.getCharAdvance(offset - start) == 0.0f) {
         // In the middle of a cluster, distribute width of cluster so that each grapheme cluster
         // gets an equal share.
         // TODO: get caret information out of font when that's available