Add support for Heretic + Hexen images

They have a widescreen fullscreen size of 560x200, instead of the old 320x200.

Author: Gaerzi

src/Graphics/SImage/SIFormat.cpp

@@ -221,6 +221,7 @@ uint32_t valid_flat_size[][3] = {
 	{ 256, 256, 1 },   // hires flat size
 	{ 320, 200, 0 },   // full screen format
 	{ 512, 512, 1 },   // hires flat size
+	{ 560, 200, 0 },   // Heretic + Hexen widescreen
 	{ 640, 400, 1 },   // SRB2 fade mask size 4
 	{ 1024, 1024, 1 }, // hires flat size
 	{ 2048, 2048, 1 }, // super hires flat size (SRB2)