Make coordinates relative to center

Relative coordinates make it easier to center the image.
The value "auto" is now replaced with simply 0.
The value "native" is replaced with "keep" for consistency.
The value is clamped, so x=-9999 will align to the left border etc.

src/config.h

@@ -14,8 +14,7 @@ enum HackBGRT_action {
  * @see struct HackBGRT_config
  */
 enum HackBGRT_coordinate {
-	HackBGRT_coord_auto = 0x10000001,
-	HackBGRT_coord_native = 0x10000002
+	HackBGRT_coord_keep = -999999
 };
 
 /**
@@ -30,6 +29,8 @@ struct HackBGRT_config {
 	int image_weight_sum;
 	int resolution_x;
 	int resolution_y;
+	int old_resolution_x;
+	int old_resolution_y;
 	const CHAR16* boot_path;
 };