Fix AGG crash when rendering extremely long horizontal lines.

pkgw · pkgw · commit c4bf0ae1f580 · 2021-01-01T12:23:19.000-05:00
When AGG rasterizes a horizontal line, it has code to check if the
computations will overflow its fixed-precision coordinate math. If an
hline is too wide, it will subdivide the line into two pieces until the
overflow is avoided.

Except a `return` statement was missing, so overflow-y code ran as well
as the subdivided code! Leading to negative coordinates and a crash.
This only happens for lines that span more than something like 32768
pixels in the X direction, so it's not something that happens in regular
usage.
diff --git a/extern/agg24-svn/include/agg_rasterizer_cells_aa.h b/extern/agg24-svn/include/agg_rasterizer_cells_aa.h
@@ -329,6 +329,7 @@ namespace agg
             int cy = (y1 + y2) >> 1;
             line(x1, y1, cx, cy);
             line(cx, cy, x2, y2);
+            return;
         }
 
         int dy = y2 - y1;

Original file line number	Diff line number	Diff line change
`@@ -329,6 +329,7 @@ namespace agg`
`329`	`329`	`int cy = (y1 + y2) >> 1;`
`330`	`330`	`line(x1, y1, cx, cy);`
`331`	`331`	`line(cx, cy, x2, y2);`
	`332`	`+ return;`
`332`	`333`	`}`
`333`	`334`
`334`	`335`	`int dy = y2 - y1;`