Fix duplicated CGB color calculation in pal sorting

And also the inverted alpha channel condition causing the wrong colors
to be treated as transparent

include/gfx/convert.hpp

@@ -9,6 +9,51 @@
 #ifndef RGBDS_GFX_CONVERT_HPP
 #define RGBDS_GFX_CONVERT_HPP
 
+#include <assert.h>
+#include <stdint.h>
+
+#include "gfx/main.hpp"
+
+struct Rgba {
+	uint8_t red;
+	uint8_t green;
+	uint8_t blue;
+	uint8_t alpha;
+
+	Rgba(uint8_t r, uint8_t g, uint8_t b, uint8_t a) : red(r), green(g), blue(b), alpha(a) {}
+	Rgba(uint32_t rgba) : red(rgba), green(rgba >> 8), blue(rgba >> 16), alpha(rgba >> 24) {}
+
+	operator uint32_t() const {
+		auto shl = [](uint8_t val, unsigned shift) { return static_cast<uint32_t>(val) << shift; };
+		return shl(red, 0) | shl(green, 8) | shl(blue, 16) | shl(alpha, 24);
+	}
+	bool operator!=(Rgba const &other) const {
+		return static_cast<uint32_t>(*this) != static_cast<uint32_t>(other);
+	}
+
+	bool isGray() const { return red == green && green == blue; }
+
+	/**
+	 * CGB colors are RGB555, so we use bit 15 to signify that the color is transparent instead
+	 * Since the rest of the bits don't matter then, we return 0x8000 exactly.
+	 */
+	static constexpr uint16_t transparent = 0b1'00000'00000'00000;
+
+	static constexpr uint8_t transparency_threshold = 5; // TODO: adjust this
+	/**
+	 * Computes the equivalent CGB color, respects the color curve depending on options
+	 */
+	uint16_t cgbColor() const {
+		if (alpha > 0xFF - transparency_threshold)
+			return transparent;
+		if (options.useColorCurve) {
+			assert(!"TODO");
+		} else {
+			return (red >> 3) | (green >> 3) << 5 | (blue >> 3) << 10;
+		}
+	}
+};
+
 void process();
 
 #endif /* RGBDS_GFX_CONVERT_HPP */