CVE-2019-6245: Out-of-bounds Write

An issue was discovered in Anti-Grain Geometry (AGG) 2.4 as used in SVG++ (aka svgpp) 1.2.3. In the function agg::cell_aa::not_equal, dx is assigned to (x2 - x1). If dx >= dx_limit, which is (16384 << poly_subpixel_shift), this function will call itself recursively. There can be a situation where (x2 - x1) is always bigger than dx_limit during the recursion, leading to continual stack consumption.