Fix RGBGFX to support -C/--color-curve in -r/--reverse mode (#2110)

Clarifies error message when RGBGFX reverse mode has `-c` mismatching `-p`.

test/gfx/reverse_color_mismatch.err

@@ -0,0 +1,3 @@
+warning: Colors reversed from the palette file do not match those specified with '-c'
+Colors specified in the palette file:         ...versus with '-c':
+[#adceefff, <none>   , <none>   , <none>   ]  [#abcdefff, <none>   , <none>   , <none>   ]

test/gfx/reverse_color_mismatch.flags

@@ -0,0 +1,3 @@
+-c #abcdef
+-s 1
+-p reverse_color_mismatch.pal

test/gfx/reverse_color_mismatch.pal

@@ -0,0 +1 @@
+5w