Implement warning diagnostic flags for RGBGFX (#1738)

include/gfx/warning.hpp

@@ -3,6 +3,29 @@
 #ifndef RGBDS_GFX_WARNING_HPP
 #define RGBDS_GFX_WARNING_HPP
 
+#include "diagnostics.hpp"
+
+enum WarningLevel {
+	LEVEL_DEFAULT,    // Warnings that are enabled by default
+	LEVEL_ALL,        // Warnings that probably indicate an error
+	LEVEL_EVERYTHING, // Literally every warning
+};
+
+enum WarningID {
+	WARNING_EMBEDDED,      // Using an embedded PNG palette without '-c embedded'
+	WARNING_TRIM_NONEMPTY, // '-x' trims nonempty tiles
+
+	NB_PLAIN_WARNINGS,
+
+	NB_WARNINGS = NB_PLAIN_WARNINGS,
+};
+
+extern Diagnostics<WarningLevel, WarningID> warnings;
+
+// Warns the user about problems that don't prevent valid graphics conversion
+[[gnu::format(printf, 2, 3)]]
+void warning(WarningID id, char const *fmt, ...);
+
 // Prints the error count, and exits with failure
 [[noreturn]]
 void giveUp();