Fix RGBGFX reversal (#1425)

* Print all OoB tilemap IDs before aborting

* Rename `nbTileInstances` to `mapSize`

* Check that reversing doesn't overflow the tile array

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Co-authored-by: ISSOtm <me@eldred.fr>

include/gfx/main.hpp

@@ -65,6 +65,10 @@ extern Options options;
  * Prints the error count, and exits with failure
  */
 [[noreturn]] void giveUp();
+/*
+ * If any error has been emitted thus far, calls `giveUp()`.
+ */
+void requireZeroErrors();
 /*
  * Prints a warning, and does not change the error count
  */