fixed sf::Image::create

Discussion thread: http://en.sfml-dev.org/forums/index.php?topic=20875.0
Basically, the issue with sf::Image::create was, that it would either
occupy space, it doesn't need, because std::vector::resize doesn't
actually shrink the vector, or reallocate in an inefficient way by
needlessly copying the whole old image over. Neither did it grant strong
exception safety guarantee because it changed the non-throwing members
(m_size) prior to doing the critical stuff (reallocating) which may
throw. Changing the order and using a temporary
(create-temporary-and-swap idiom; see http://www.gotw.ca/gotw/059.htm)
fixes both of these problems.

src/SFML/Graphics/Image.cpp

@@ -66,16 +66,12 @@ void Image::create(unsigned int width, unsigned int height, const Color& color)
 {
     if (width && height)
     {
-        // Assign the new size
-        m_size.x = width;
-        m_size.y = height;
-
-        // Resize the pixel buffer
-        m_pixels.resize(width * height * 4);
+        // Create a new pixel buffer first for exception safety's sake
+        std::vector<Uint8> newPixels(width * height * 4);
     
         // Fill it with the specified color
-        Uint8* ptr = &m_pixels[0];
-        Uint8* end = ptr + m_pixels.size();
+        Uint8* ptr = &newPixels[0];
+        Uint8* end = ptr + newPixels.size();
         while (ptr < end)
         {
             *ptr++ = color.r;
@@ -83,13 +79,22 @@ void Image::create(unsigned int width, unsigned int height, const Color& color)
             *ptr++ = color.b;
             *ptr++ = color.a;
         }
+    
+        // Commit the new pixel buffer
+        m_pixels.swap(newPixels);
+        
+        // Assign the new size
+        m_size.x = width;
+        m_size.y = height;
     }
     else
     {
-        // Create an empty image
+        // Dump the pixel buffer
+        std::vector<Uint8>().swap(m_pixels);
+        
+        // Assign the new size
         m_size.x = 0;
         m_size.y = 0;
-        m_pixels.clear();
     }
 }
 
@@ -99,21 +104,24 @@ void Image::create(unsigned int width, unsigned int height, const Uint8* pixels)
 {
     if (pixels && width && height)
     {
+        // Create a new pixel buffer first for exception safety's sake
+        std::vector<Uint8> newPixels(pixels, pixels + width * height * 4);
+        
+        // Commit the new pixel buffer
+        m_pixels.swap(newPixels);
+        
         // Assign the new size
         m_size.x = width;
         m_size.y = height;
-
-        // Copy the pixels
-        std::size_t size = width * height * 4;
-        m_pixels.resize(size);
-        std::memcpy(&m_pixels[0], pixels, size); // faster than vector::assign
     }
     else
     {
-        // Create an empty image
+        // Dump the pixel buffer
+        std::vector<Uint8>().swap(m_pixels);
+        
+        // Assign the new size
         m_size.x = 0;
         m_size.y = 0;
-        m_pixels.clear();
     }
 }