Add config option working around buggy graphics drivers

"subImageFix=true" should fix missing text on radeonsi fglrx
as well as most mobile drivers. Also fixes tileset atlas on
mobile.

mkxp.conf.sample

@@ -94,6 +94,15 @@
 # solidFonts=false
 
 
+# Work around buggy graphics drivers which don't
+# properly synchronize texture access, most
+# apparent when text doesn't show up or the map
+# tileset doesn't render at all
+# (default: disabled)
+#
+# subImageFix=false
+
+
 # Set the base path of the game to '/path/to/game'
 # (default: executable directory)
 #