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Reject colors with ambiguous alpha channel

--- a/include/gfx/rgba.hpp

+++ b/include/gfx/rgba.hpp

@@ -9,6 +9,7 @@

 #ifndef RGBDS_GFX_RGBA_HPP

 #define RGBDS_GFX_RGBA_HPP

+#include <cstdint>

 #include <stdint.h>

 struct Rgba {

@@ -50,12 +51,10 @@

 	 */

 	static constexpr uint16_t transparent = 0b1'00000'00000'00000;

-	/**

-	 * All alpha values strictly below this will be considered transparent

-	 */

-	static constexpr uint8_t opacity_threshold = 0xF0; // TODO: adjust this

-	// TODO: also a transparency threshold, and error out on "middle" values

-	bool isTransparent() const { return alpha < opacity_threshold; }

+	static constexpr uint8_t transparency_threshold = 0x10;

+	bool isTransparent() const { return alpha < transparency_threshold; }

+	static constexpr uint8_t opacity_threshold = 0xF0;

+	bool isOpaque() const { return alpha >= opacity_threshold; }

 	/**

 	 * Computes the equivalent CGB color, respects the color curve depending on options

 	 */

--- a/src/gfx/process.cpp

+++ b/src/gfx/process.cpp

@@ -323,10 +323,22 @@

 		// assigned, and conflicts always occur between that and another color.

 		// For the same reason, we don't need to worry about order, either.

 		std::vector<std::tuple<uint32_t, uint32_t>> conflicts;

+		// Holds colors whose alpha value is ambiguous

+		std::vector<uint32_t> indeterminates;

 		// Assign a color to the given position, and register it in the image palette as well

-		auto assignColor = [this, &conflicts](png_uint_32 x, png_uint_32 y, Rgba &&color) {

-			if (Rgba const *other = colors.registerColor(color); other) {

+		auto assignColor = [this, &conflicts, &indeterminates](png_uint_32 x, png_uint_32 y,

+		                                                       Rgba &&color) {

+			if (!color.isTransparent() && !color.isOpaque()) {

+				uint32_t css = color.toCSS();

+				if (std::find(indeterminates.begin(), indeterminates.end(), css)

+				    == indeterminates.end()) {

+					error(

+					    "Color #%08x is neither transparent (alpha < %u) nor opaque (alpha >= %u)",

+					    css, Rgba::transparency_threshold, Rgba::opacity_threshold);

+					indeterminates.push_back(css);

+				}

+			} else if (Rgba const *other = colors.registerColor(color); other) {

 				std::tuple conflicting{color.toCSS(), other->toCSS()};

 				// Do not report combinations twice

 				if (std::find(conflicts.begin(), conflicts.end(), conflicting) == conflicts.end()) {

--- a/src/gfx/rgba.cpp

+++ b/src/gfx/rgba.cpp

@@ -33,6 +33,7 @@

 	if (isTransparent()) {

 		return transparent;

 	}

+	assert(isOpaque());

 	uint8_t r = red, g = green, b = blue;

 	if (options.useColorCurve) {