Cancel -U option

As it turns out, it is really difficult to implement, and can be dealt with later.

include/gfx/main.hpp

@@ -52,7 +52,6 @@ struct Options {
 	std::string palmap{}; // -q, -Q
 	uint8_t nbColorsPerPal = 0; // -s; 0 means "auto" = 1 << bitDepth;
 	std::string tilemap{}; // -t, -T
-	std::array<uint16_t, 2> unitSize{1, 1}; // -U (in tiles)
 	uint64_t trim = 0; // -x
 
 	std::string input{}; // positional arg