Rewrite RGBFIX

- Make it work inside pipelines
- Add RGBFIX tests to the suite
- Be more flexible in accepted MBC names
- Add warnings for dangerous or nonsensical input params
- Improve man page

test/fix/verify-pad.flags

@@ -0,0 +1,4 @@
+-vp 69
+Check that the global checksum is correctly affected by padding:
+Padding adds extra bytes (carefully picked *not* to be 0, or other values),
+which must be properly accounted for.