Handle correct data offset in raw decoder

There is often buffered data ahead of the pixel data so we need to take
this in account when making sure pixels are opaque.

core/decoders/raw.js

@@ -51,7 +51,7 @@ export default class RawDecoder {
 
         // Max sure the image is fully opaque
         for (let i = 0; i < pixels; i++) {
-            data[i * 4 + 3] = 255;
+            data[index + i * 4 + 3] = 255;
         }
 
         display.blitImage(x, curY, width, currHeight, data, index);