Fix issues with loading images with alpha disabled.

These issues are fixed:
* loadPNG gave RGBA surfaces unconditionally. Now it gives RGB surfaces if
  its second parameter, defaulting to true, is false. This fixes adding per-
  surface alpha values, because an RGBA surface ignores its per-surface value.
* Surface::loadImage is updated to receive a third parameter, defaulting to
  true, to determine whether alpha is loaded.
* SurfaceCollection::defaultAlpha was never used.
* SurfaceCollection::defaultAlpha defaulted to false, so even if it were used,
  it would have loaded images without alpha.

src/imageio.cpp

@@ -24,7 +24,7 @@ static void __readFromOpk(png_structp png_ptr, png_bytep ptr, png_size_t length)
 }
 #endif
 
-SDL_Surface *loadPNG(const std::string &path) {
+SDL_Surface *loadPNG(const std::string &path, bool loadAlpha) {
 	// Declare these with function scope and initialize them to NULL,
 	// so we can use a single cleanup block at the end of the function.
 	SDL_Surface *surface = NULL;
@@ -134,10 +134,10 @@ SDL_Surface *loadPNG(const std::string &path) {
 		goto cleanup;
 	}
 
-	// Allocate ARGB surface to hold the image.
+	// Allocate [A]RGB surface to hold the image.
 	surface = SDL_CreateRGBSurface(
 		SDL_SWSURFACE | SDL_SRCALPHA, width, height, 32,
-		0x00FF0000, 0x0000FF00, 0x000000FF, 0xFF000000
+		0x00FF0000, 0x0000FF00, 0x000000FF, loadAlpha ? 0xFF000000 : 0x00000000
 		);
 	if (!surface) {
 		// Failed to create surface, probably out of memory.

src/imageio.h

@@ -7,6 +7,6 @@ struct SDL_Surface;
 
 /** Loads an image from a PNG file into a newly allocated 32bpp RGBA surface.
   */
-SDL_Surface *loadPNG(const std::string &path);
+SDL_Surface *loadPNG(const std::string &path, bool loadAlpha = true);
 
 #endif

src/surface.cpp

@@ -55,14 +55,14 @@ Surface *Surface::emptySurface(int width, int height) {
 	return new Surface(raw, true);
 }
 
-Surface *Surface::loadImage(const string &img, const string &skin) {
+Surface *Surface::loadImage(const string &img, const string &skin, bool loadAlpha) {
 	string skinpath;
 	if (!skin.empty() && !img.empty() && img[0]!='/')
 	  skinpath = SurfaceCollection::getSkinFilePath(skin, img);
 	else
 	  skinpath = img;
 
-	SDL_Surface *raw = loadPNG(skinpath);
+	SDL_Surface *raw = loadPNG(skinpath, loadAlpha);
 	if (!raw) {
 		ERROR("Couldn't load surface '%s'\n", img.c_str());
 		return NULL;

src/surface.h

@@ -41,7 +41,7 @@ public:
 	static Surface *openOutputSurface(int width, int height, int bitsperpixel);
 	static Surface *emptySurface(int width, int height);
 	static Surface *loadImage(const std::string &img,
-			const std::string &skin="");
+			const std::string &skin="", bool loadAlpha=true);
 
 	Surface(Surface *s);
 	~Surface();

src/surfacecollection.cpp

@@ -119,7 +119,7 @@ Surface *SurfaceCollection::add(const string &path) {
 	}
 
 	DEBUG("Adding surface: '%s'\n", path.c_str());
-	Surface *s = Surface::loadImage(filePath);
+	Surface *s = Surface::loadImage(filePath, "", defaultAlpha);
 	if (s != NULL) {
 		surfaces[path] = s;
 	}

src/surfacecollection.h

@@ -42,7 +42,7 @@ public:
 	static std::string getSkinFilePath(const std::string &skin, const std::string &file, bool useDefault = true);
 	static std::string getSkinPath(const std::string &skin);
 
-	bool defaultAlpha;
+	bool defaultAlpha = true;
 	void debug();
 
 	Surface *add(Surface *s, const std::string &path);