Add an [invert:<mode> texture modifier

Inverts the given channels of the base image.
Mode may contain the characters "r", "g", "b", "a".
Only the channels that are mentioned in the mode string will be inverted.

doc/lua_api.txt

@@ -318,6 +318,15 @@ Example:
 
     default_sandstone.png^[opacity:127
 
+#### `[invert:<mode>`
+Inverts the given channels of the base image.
+Mode may contain the characters "r", "g", "b", "a".
+Only the channels that are mentioned in the mode string will be inverted.
+
+Example:
+
+	default_apple.png^[invert:rgb
+
 #### `[brighten`
 Brightens the texture.
 

src/client/tile.cpp

@@ -1783,9 +1783,48 @@ bool TextureSource::generateImagePart(std::string part_of_name,
 			for (u32 y = 0; y < dim.Height; y++)
 			for (u32 x = 0; x < dim.Width; x++)
 			{
-				video::SColor c = baseimg->getPixel(x,y);
+				video::SColor c = baseimg->getPixel(x, y);
 				c.setAlpha(floor((c.getAlpha() * ratio) / 255 + 0.5));
-				baseimg->setPixel(x,y,c);
+				baseimg->setPixel(x, y, c);
+			}
+		}
+		/*
+			[invert:mode
+			Inverts the given channels of the base image.
+			Mode may contain the characters "r", "g", "b", "a".
+			Only the channels that are mentioned in the mode string
+			will be inverted.
+		*/
+		else if (str_starts_with(part_of_name, "[invert:")) {
+			if (baseimg == NULL) {
+				errorstream << "generateImagePart(): baseimg == NULL "
+						<< "for part_of_name=\"" << part_of_name
+						<< "\", cancelling." << std::endl;
+				return false;
+			}
+
+			Strfnd sf(part_of_name);
+			sf.next(":");
+
+			std::string mode = sf.next("");
+			u32 mask = 0;
+			if (mode.find("a") != std::string::npos)
+				mask |= 0xff000000UL;
+			if (mode.find("r") != std::string::npos)
+				mask |= 0x00ff0000UL;
+			if (mode.find("g") != std::string::npos)
+				mask |= 0x0000ff00UL;
+			if (mode.find("b") != std::string::npos)
+				mask |= 0x000000ffUL;
+
+			core::dimension2d<u32> dim = baseimg->getDimension();
+
+			for (u32 y = 0; y < dim.Height; y++)
+			for (u32 x = 0; x < dim.Width; x++)
+			{
+				video::SColor c = baseimg->getPixel(x, y);
+				c.color ^= mask;	
+				baseimg->setPixel(x, y, c);
 			}
 		}
 		else