Fix alpha depth check to return the right value for rgba images.

Not part of build.  r=pavlov

modules/libpr0n/decoders/png/nsPNGDecoder.cpp

@@ -302,12 +302,8 @@ nsPNGDecoder::info_callback(png_structp png_ptr, png_infop info_ptr)
   PRInt32 alpha_bits = 1;
 
   if (channels > 3) {
-
-    if (color_type || PNG_COLOR_MASK_ALPHA) {
     /* check if alpha is coming from a tRNS chunk and is binary */
     if (num_trans) {
-        alpha_bits = 1;
-
       /* if it's not a indexed color image, tRNS means binary */
       if (color_type == PNG_COLOR_TYPE_PALETTE) {
         for (int i=0; i<num_trans; i++) {
@@ -316,12 +312,9 @@ nsPNGDecoder::info_callback(png_structp png_ptr, png_infop info_ptr)
             break;
           }
         }
-        } else {
-          alpha_bits = 8/*png_ptr->pixel_depth*/;   /* 8 */
-        }
       }
     } else {
-      alpha_bits = 1;
+      alpha_bits = 8;
     }
   }