Fix "out of bounds" undefined behavior

This is detected with clang and -fsanitizer=bounds.

Out-of-bound addressing array of fixed size is undefined
behavior in both C and C++. It is so even if underlying
memory is properly allocated. To avoid undefined behavior
in cases like this C99 added feature called "flexible array member"
when size of array is omitted.

Unfortunately it's not C++ feature. However both clang++ and
g++ this and this seems the only reasonable way
to avoid undefined behavior without significant rewriting such code.

Author: vitalybuka

extern/agg24-svn/include/agg_pixfmt_rgba.h

@@ -1521,7 +1521,7 @@ namespace agg
         };
         struct pixel_type
         {
-            value_type c[num_components];
+            value_type c[];
 
             void set(value_type r, value_type g, value_type b, value_type a)
             {
@@ -1733,12 +1733,10 @@ namespace agg
                                    unsigned len, 
                                    const color_type& c)
         {
-            pixel_type v;
-            v.set(c);
             pixel_type* p = pix_value_ptr(x, y, len);
             do
             {
-                *p = v;
+                p->set(c);
                 p = p->next();
             }
             while (--len);
@@ -1750,11 +1748,9 @@ namespace agg
                                    unsigned len, 
                                    const color_type& c)
         {
-            pixel_type v;
-            v.set(c);
             do
             {
-                *pix_value_ptr(x, y++, 1) = v;
+                pix_value_ptr(x, y++, 1)->set(c);
             }
             while (--len);
         }
@@ -1770,11 +1766,9 @@ namespace agg
                 pixel_type* p = pix_value_ptr(x, y, len);
                 if (c.is_opaque() && cover == cover_mask)
                 {
-                    pixel_type v;
-                    v.set(c);
                     do
                     {
-                        *p = v;
+                        p->set(c);
                         p = p->next();
                     }
                     while (--len);
@@ -1814,11 +1808,9 @@ namespace agg
             {
                 if (c.is_opaque() && cover == cover_mask)
                 {
-                    pixel_type v;
-                    v.set(c);
                     do
                     {
-                        *pix_value_ptr(x, y++, 1) = v;
+                        pix_value_ptr(x, y++, 1)->set(c);
                     }
                     while (--len);
                 }