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

include/helpers.h

@@ -32,6 +32,58 @@
 	static inline _Noreturn unreachable_(void) {}
 #endif
 
+// Use builtins whenever possible, and shim them otherwise
+#ifdef __GNUC__ // GCC or compatible
+	#define ctz __builtin_ctz
+	#define clz __builtin_clz
+
+#elif defined(_MSC_VER)
+	#include <assert.h>
+	#include <intrin.h>
+	#pragma intrinsic(_BitScanReverse, _BitScanForward)
+	static inline int ctz(unsigned int x)
+	{
+		unsigned long cnt;
+
+		assert(x != 0);
+		_BitScanForward(&cnt, x);
+		return cnt;
+	}
+	static inline int clz(unsigned int x)
+	{
+		unsigned long cnt;
+
+		assert(x != 0);
+		_BitScanReverse(&cnt, x);
+		return 31 - cnt;
+	}
+
+#else
+	// FIXME: these are rarely used, and need testing...
+	#include <limits.h>
+	static inline int ctz(unsigned int x)
+	{
+		int cnt = 0;
+
+		while (!(x & 1)) {
+			x >>= 1;
+			cnt++;
+		}
+		return cnt;
+	}
+
+	static inline int clz(unsigned int x)
+	{
+		int cnt = 0;
+
+		while (x <= UINT_MAX / 2) {
+			x <<= 1;
+			cnt++;
+		}
+		return cnt;
+	}
+#endif
+
 // Macros for stringification
 #define STR(x) #x
 #define EXPAND_AND_STR(x) STR(x)