Fix line drawing path to use proportional distance from center

boundaryWidth and boundaryLength are now distance from center, instead of
distance from edge, in order to avoid unnecessary calculation in the shader. New
calculation of these matches drawAARect.

Change-Id: I1c7f576a97a6f0ce00d521661fd56fb62e05e6f2

Author: ChrisCraik

libs/hwui/OpenGLRenderer.cpp

@@ -1937,7 +1937,7 @@ status_t OpenGLRenderer::drawLines(float* points, int count, SkPaint* paint) {
         // This value is used in the fragment shader to determine how to fill fragments.
         // We will need to calculate the actual width proportion on each segment for
         // scaled non-hairlines, since the boundary proportion may differ per-axis when scaled.
-        float boundaryWidthProportion = 1 / (2 * halfStrokeWidth);
+        float boundaryWidthProportion = .5 - 1 / (2 * halfStrokeWidth);
         setupDrawAALine((void*) aaVertices, widthCoords, lengthCoords,
                 boundaryWidthProportion, widthSlot, lengthSlot);
     }
@@ -2001,9 +2001,9 @@ status_t OpenGLRenderer::drawLines(float* points, int count, SkPaint* paint) {
                 abVector.x *= inverseScaleX;
                 abVector.y *= inverseScaleY;
                 float abLength = abVector.length();
-                boundaryLengthProportion = abLength / (length + abLength);
+                boundaryLengthProportion = .5 - abLength / (length + abLength);
             } else {
-                boundaryLengthProportion = .5 / (length + 1);
+                boundaryLengthProportion = .5 - .5 / (length + 1);
             }
 
             abVector /= 2;