Rename to prevent conflict with png_image from libpng16

http://bugs.gentoo.org/464762

--- rwpng.c
+++ rwpng.c
@@ -213,7 +213,7 @@
 }
 
 
-pngquant_error rwpng_write_image_init(png_image *mainprog_ptr, png_structpp png_ptr_p, png_infopp info_ptr_p, FILE *outfile)
+pngquant_error rwpng_write_image_init(rwpng_png_image *mainprog_ptr, png_structpp png_ptr_p, png_infopp info_ptr_p, FILE *outfile)
 {
     /* could also replace libpng warning-handler (final NULL), but no need: */
 
@@ -277,7 +277,7 @@
     png_structp png_ptr;
     png_infop info_ptr;
 
-    pngquant_error retval = rwpng_write_image_init((png_image*)mainprog_ptr, &png_ptr, &info_ptr, outfile);
+    pngquant_error retval = rwpng_write_image_init((rwpng_png_image*)mainprog_ptr, &png_ptr, &info_ptr, outfile);
     if (retval) return retval;
 
     // Palette images generally don't gain anything from filtering
@@ -321,7 +321,7 @@
     png_structp png_ptr;
     png_infop info_ptr;
 
-    pngquant_error retval = rwpng_write_image_init((png_image*)mainprog_ptr, &png_ptr, &info_ptr, outfile);
+    pngquant_error retval = rwpng_write_image_init((rwpng_png_image*)mainprog_ptr, &png_ptr, &info_ptr, outfile);
     if (retval) return retval;
 
     rwpng_set_gamma(info_ptr, png_ptr, mainprog_ptr->gamma);
@@ -344,7 +344,7 @@
 
 static void rwpng_error_handler(png_structp png_ptr, png_const_charp msg)
 {
-    png_image  *mainprog_ptr;
+    rwpng_png_image  *mainprog_ptr;
 
     /* This function, aside from the extra step of retrieving the "error
      * pointer" (below) and the fact that it exists within the application
--- rwpng.h
+++ rwpng.h
@@ -80,7 +80,7 @@
     jmp_buf jmpbuf;
     png24_image png24;
     png8_image png8;
-} png_image;
+} rwpng_png_image;
 
 /* prototypes for public functions in rwpng.c */