Made the png_loader::get_image() function a little faster.

dlib/image_loader/png_loader.h

@@ -40,17 +40,26 @@ namespace dlib
 #endif
 
             t.set_size( height_, width_ );
+
+
+            if (is_gray())
+            {
                 for ( unsigned n = 0; n < height_;n++ )
                 {
                     const unsigned char* v = get_row( n );
                     for ( unsigned m = 0; m < width_;m++ )
-                {
-                    if ( is_gray() )
                     {
                         unsigned char p = v[m];
                         assign_pixel( t[n][m], p );
                     }
-                    else if ( is_rgb() )
+                }
+            }
+            else if (is_rgb())
+            {
+                for ( unsigned n = 0; n < height_;n++ )
+                {
+                    const unsigned char* v = get_row( n );
+                    for ( unsigned m = 0; m < width_;m++ )
                     {
                         rgb_pixel p;
                         p.red = v[m*3];
@@ -58,7 +67,14 @@ namespace dlib
                         p.blue = v[m*3+2];
                         assign_pixel( t[n][m], p );
                     }
-                    else if ( is_rgba() )
+                }
+            }
+            else if (is_rgba())
+            {
+                for ( unsigned n = 0; n < height_;n++ )
+                {
+                    const unsigned char* v = get_row( n );
+                    for ( unsigned m = 0; m < width_;m++ )
                     {
                         rgb_alpha_pixel p;
                         p.red = v[m*4];