sw_engine: split long lines to avoid overflow

@Issue: https://github.com/thorvg/thorvg/issues/2651

src/renderer/sw_engine/tvgSwCommon.h

@@ -488,6 +488,7 @@ SwFixed mathAtan(const SwPoint& pt);
 SwFixed mathCos(SwFixed angle);
 SwFixed mathSin(SwFixed angle);
 void mathSplitCubic(SwPoint* base);
+void mathSplitLine(SwPoint* base);
 SwFixed mathDiff(SwFixed angle1, SwFixed angle2);
 SwFixed mathLength(const SwPoint& pt);
 int mathCubicAngle(const SwPoint* base, SwFixed& angleIn, SwFixed& angleMid, SwFixed& angleOut);

src/renderer/sw_engine/tvgSwMath.cpp

@@ -242,6 +242,15 @@ void mathSplitCubic(SwPoint* base)
 }
 
 
+void mathSplitLine(SwPoint* base)
+{
+    base[2] = base[1];
+
+    base[1].x = (base[0].x + base[1].x) >> 1;
+    base[1].y = (base[0].y + base[1].y) >> 1;
+}
+
+
 SwFixed mathDiff(SwFixed angle1, SwFixed angle2)
 {
     auto delta = angle2 - angle1;

src/renderer/sw_engine/tvgSwRle.cpp

@@ -235,6 +235,7 @@ struct RleWorker
     SwPoint pos;
 
     SwPoint bezStack[32 * 3 + 1];
+    SwPoint lineStack[32 + 1];
     int levStack[32];
 
     SwOutline* outline;
@@ -513,8 +514,23 @@ static void _lineTo(RleWorker& rw, const SwPoint& to)
         return;
     }
 
-    auto diff = to - rw.pos;
-    auto f1 = rw.pos - SUBPIXELS(e1);
+    auto line = rw.lineStack;
+    line[0] = to;
+    line[1] = rw.pos;
+
+    while (true) {
+        auto diff = line[0] - line[1];
+        auto L = HYPOT(diff);
+
+        if (L > SHRT_MAX) {
+            mathSplitLine(line);
+            ++line;
+            continue;
+        }
+        e1 = TRUNC(line[1]);
+        e2 = TRUNC(line[0]);
+
+        auto f1 = line[1] - SUBPIXELS(e1);
         SwPoint f2;
 
         //inside one cell
@@ -558,12 +574,7 @@ static void _lineTo(RleWorker& rw, const SwPoint& to)
             /* The fundamental value `prod' determines which side and the  */
             /* exact coordinate where the line exits current cell.  It is  */
             /* also easily updated when moving from one cell to the next.  */
-        size_t idx = 0;
             do {
-            if (++idx > 1000000000) {
-                TVGERR("SW_ENGINE", "Iteration limit reached during RLE generation. The resulting render may be incorrect.");
-                break;
-            }
                 auto px = diff.x * ONE_PIXEL;
                 auto py = diff.y * ONE_PIXEL;
 
@@ -606,10 +617,13 @@ static void _lineTo(RleWorker& rw, const SwPoint& to)
             } while(e1 != e2);
         }
 
-    f2 = {to.x - SUBPIXELS(e2.x), to.y - SUBPIXELS(e2.y)};
+        f2 = {line[0].x - SUBPIXELS(e2.x), line[0].y - SUBPIXELS(e2.y)};
         rw.cover += (f2.y - f1.y);
         rw.area += (f2.y - f1.y) * (f1.x + f2.x);
-    rw.pos = to;
+        rw.pos = line[0];
+
+        if (line-- == rw.lineStack) return;
+    }
 }