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/README.md

@@ -0,0 +1,16 @@
+# RGBFIX tests
+
+These tests check that RGBFIX behaves properly.
+
+## Structure of a test
+
+- `test.bin`: The file passed as input to RGBFIX.
+- `test.flags`: The command-line flags passed to RGBFIX's invocation.
+  Actually, only the first line is considered; the rest of the file may contain comments about the test.
+- `test.gb`: The expected output.
+  May not exist, generally when the test expects an error, in which case the comparison is skipped.
+- `test.err`: The expected error output.
+
+## Special tests
+
+- `noexist.err` is the expected error output when RGBFIX is given a non-existent input file.