Add error for loading non 24-bit bitmaps

data/language/en-GB.txt

@@ -4362,6 +4362,7 @@ STR_6050    :Error reading bitmap
 STR_6051    :The width and height need to match
 STR_6052    :The heightmap is too big, and will be cut off
 STR_6053    :The heightmap cannot be normalized
+STR_6054    :Only 24-bit bitmaps are supported
 
 #############
 # Scenarios #

src/openrct2/localisation/string_ids.h

@@ -3712,6 +3712,7 @@ enum {
 	STR_ERROR_WIDTH_AND_HEIGHT_DO_NOT_MATCH = 6051,
 	STR_ERROR_HEIHGT_MAP_TOO_BIG = 6052,
 	STR_ERROR_CANNOT_NORMALIZE = 6053,
+	STR_ERROR_24_BIT_BITMAP = 6054,
 
 	// Have to include resource strings (from scenarios and objects) for the time being now that language is partially working
 	STR_COUNT = 32768

src/openrct2/world/mapgen.c

@@ -816,6 +816,13 @@ bool mapgen_load_heightmap(const utf8 *path)
 		numChannels = bitmap->format->BytesPerPixel;
 		pitch = bitmap->pitch;
 
+		if (numChannels < 3 || bitmap->format->BitsPerPixel < 24)
+		{
+			window_error_open(STR_HEIGHT_MAP_ERROR, STR_ERROR_24_BIT_BITMAP);
+			SDL_FreeSurface(bitmap);
+			return false;
+		}
+
 		// Copy pixels over, then discard the surface
 		SDL_LockSurface(bitmap);
 		pixels = malloc(height * bitmap->pitch);