SkScan_Antihair: assert in debug and potentialy lost last pixels

src/core/SkScan_Antihair.cpp

Index: src/core/SkScan_Antihair.cpp
diff --git a/src/core/SkScan_Antihair.cpp b/src/core/SkScan_Antihair.cpp
index beddf6320297a5a5870997955a664a29e9653f22..b14e12bab8c8d3157ae37ec73d3b67ca2e3c2d43 100644
--- a/src/core/SkScan_Antihair.cpp
+++ b/src/core/SkScan_Antihair.cpp
@@ -847,7 +847,11 @@ static void inner_scanline(FDot8 L, int top, FDot8 R, U8CPU alpha,
     SkASSERT(L < R);
 
     if ((L >> 8) == ((R - 1) >> 8)) {  // 1x1 pixel
-        blitter->blitV(L >> 8, top, 1, InvAlphaMul(alpha, R - L));
+        FDot8 widClamp = R - L;
+        // border case clamp 256 to 255 instead of going through call_hline_blitter
+        // see skbug/4406
+        widClamp = widClamp - (widClamp >> 8);
+        blitter->blitV(L >> 8, top, 1, InvAlphaMul(alpha, widClamp));
         return;
     }