Bug 1425294 - Use a hardcoded background color for mozscreenshots so the APNG will not have transparency. r=MattN

MozReview-Commit-ID: CUwTQdXPBg6

--HG--
extra : rebase_source : ad8fccd68da35132e93052148cf289a83ca13d1c

browser/tools/mozscreenshots/mozscreenshots/extension/TestRunner.jsm

@@ -378,6 +378,9 @@ this.TestRunner = {
         canvas.height = bounds.height;
         const ctx = canvas.getContext("2d");
 
+        ctx.fillStyle = "hotpink";
+        ctx.fillRect(0, 0, bounds.width, bounds.height);
+
         for (const rect of rects) {
           rect.left = Math.max(0, rect.left);
           rect.right = Math.min(img.naturalWidth, rect.right);