Revert "ImBuf: remove ".tx" extension from the list of supported images"

This reverts commit cf55a0acdc.

While this was added with the commit that added support for tile cache,
keep the extension check as Blender can load them.

source/blender/imbuf/intern/util.cc

@@ -26,7 +26,7 @@
 #define UTIL_DEBUG 0
 
 const char *imb_ext_image[] = {
-    ".png",  ".tga", ".bmp", ".jpg", ".jpeg", ".sgi", ".rgb", ".rgba", ".tif", ".tiff",
+    ".png",  ".tga", ".bmp", ".jpg", ".jpeg", ".sgi", ".rgb", ".rgba", ".tif", ".tiff", ".tx",
 #ifdef WITH_OPENJPEG
     ".jp2",  ".j2c",
 #endif