Fix a flaky/random-seed rgbgfx_test test failure

Commit 21eaa2d2cc missed some
intentional unsigned underflow that is needed to set `nbTilesInBank`.
Without it, some of the randomly-seeded RGBGFX tests, such as
`./rgbgfx_test seed8.bin -b 207 -N 54,256`, can fail with "Color
mismatch after round-tripping" errors.

src/gfx/reverse.cpp

@@ -320,9 +320,11 @@ void reverse() {
 					);
 				}
 			} else {
-				if (uint8_t tileID = (*tilemap)[index];
-				    tileID >= nbTilesInBank[bank] + options.baseTileIDs[bank]) {
-					nbTilesInBank[bank] = tileID - options.baseTileIDs[bank] + 1;
+				// The unsigned underflow for `tileOfs` is intentional, since a nonzero
+				// base tile ID may overflow and continue with IDs from 0.
+				if (uint8_t tileOfs = (*tilemap)[index] - options.baseTileIDs[bank];
+				    tileOfs >= nbTilesInBank[bank]) {
+					nbTilesInBank[bank] = tileOfs + 1;
 				}
 			}
 		}