AGS: Fix fully transparent savegame screenshots

If On A Winter's Night, Four Travelers was such as game that
generated fully transparent screenshots. I am not completely
sure why it happens, and the easiest solution was to reset
the alpha byte to 0xff.

engines/ags/engine/ac/game.cpp

@@ -922,6 +922,7 @@ void create_savegame_screenshot(Bitmap *&screenShot) {
 			quit("!Invalid game.screenshot_width/height, must be from 16x16 to screen res");
 
 		screenShot = CopyScreenIntoBitmap(usewid, usehit);
+		screenShot->GetAllegroBitmap()->makeOpaque();
 
 		// Restore original screen
 		_G(debug_flags) = old_flags;

engines/ags/lib/allegro/surface.cpp

@@ -51,6 +51,20 @@ int BITMAP::getpixel(int x, int y) const {
 		return *(const uint32 *)pixel;
 }
 
+void BITMAP::makeOpaque() {
+	if (format.aBits() == 0)
+		return;
+	assert(format.bytesPerPixel == 4);
+	uint32 alphaMask = format.ARGBToColor(0xff, 0, 0, 0);
+
+	unsigned char *pixels = getPixels();
+	for (int y = 0 ; y < h ; ++y, pixels += pitch) {
+		uint32 *data = (uint32*)pixels;
+		for (int x = 0 ; x < w ; ++x, ++data)
+			(*data) |= alphaMask;
+	}
+}
+
 void BITMAP::circlefill(int x, int y, int radius, int color) {
 	int cx = 0;
 	int cy = radius;

engines/ags/lib/allegro/surface.h

@@ -71,6 +71,8 @@ public:
 		_owner->clear();
 	}
 
+	void makeOpaque();
+
 	/**
 	 * Draws a solid filled in circle
 	 */