fix linear gradient assert, by explicitly clamping

src/effects/gradients/SkLinearGradient.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkLinearGradient.h"
 
 static const float kInv255Float = 1.0f / 255;
@@ skipping 650 matching lines @@
         const Sk4f dc = diffc * dx4 * Sk4f(scale);
 
         int n;
         if (dx_is_pos) {
             n = SkTMin((int)((p1 - fx) * invDx) + 1, count);
         } else {
             n = SkTMin((int)((p0 - fx) * invDx) + 1, count);
         }
 
         fx += n * dx;
-        count -= n;
-        SkASSERT(count >= 0);
+        // fx should now outside of the p0..p1 interval. However, due to float precision loss,
+        // its possible that fx is slightly too small/large, so we clamp it.
         if (dx_is_pos) {
-            SkASSERT(0 == count || fx >= p1);
+            fx = SkTMax(fx, p1);
         } else {
-            SkASSERT(0 == count || fx <= p0);
+            fx = SkTMin(fx, p0);
         }
 
         ramp<apply_alpha>(dstC, n, c, dc, dither0, dither1);
         dstC += n;
         SkASSERT(dstC <= endDstC);
 
         if (n & 1) {
             SkTSwap(dither0, dither1);
         }
+
+        count -= n;
+        SkASSERT(count >= 0);
     }
 }
 
 void SkLinearGradient::LinearGradientContext::shade4_clamp(int x, int y, SkPMColor dstC[],
                                                            int count) {
     SkASSERT(count > 0);
     SkASSERT(kLinear_MatrixClass == fDstToIndexClass);
 
     SkPoint srcPt;
     fDstToIndexProc(fDstToIndex, x + SK_ScalarHalf, y + SK_ScalarHalf, &srcPt);
@@ skipping 37 matching lines @@
         }
     } else {
         if (fApplyAlphaAfterInterp) {
             this->shade4_dx_clamp<true, false>(dstC, count, fx, dx, invDx, dither);
         } else {
             this->shade4_dx_clamp<false, false>(dstC, count, fx, dx, invDx, dither);
         }
     }
 }