Implement base palette ID

include/gfx/main.hpp

@@ -43,6 +43,7 @@ struct Options {
 		uint32_t right() const { return left + width * 8; }
 		uint32_t bottom() const { return top + height * 8; }
 	} inputSlice{0, 0, 0, 0};                          // -L (margins in clockwise order, like CSS)
+	uint8_t basePalID = 0;                             // -l
 	std::array<uint16_t, 2> maxNbTiles{UINT16_MAX, 0}; // -N
 	uint16_t nbPalettes = 8;                           // -n
 	std::string output{};                              // -o