Bug 1471583: Don't record a successful draw result if the image is not complete yet. r=tnikkel

I've tried to unsuccessfully reproduce the same kind of failures locally under
rr with this patch applied, so that's a good sign.

This should fix the case where we don't invalidate from AttemptPartialUpdate
while trying to sync-decode.

This is a clear part of the problem, though not sure yet if it'll fix all the
issues we see in these tests. In any case I want to investigate those
separately.

Differential Revision: https://phabricator.services.mozilla.com/D2068

MozReview-Commit-ID: H7OEvbQUues

layout/painting/nsImageRenderer.cpp

@@ -123,13 +123,13 @@ nsImageRenderer::PrepareImage()
 
   if (!mImage->IsComplete()) {
     // Make sure the image is actually decoding.
-    bool frameComplete = mImage->StartDecoding();
+    mImage->StartDecoding();
 
     // Check again to see if we finished.
     // We cannot prepare the image for rendering if it is not fully loaded.
-    // Special case: If we requested a sync decode and the image has loaded, push
-    // on through because the Draw() will do a sync decode then.
-    if (!(frameComplete || mImage->IsComplete()) &&
+    // Special case: If we requested a sync decode and the image has loaded,
+    // push on through because the Draw() will do a sync decode then.
+    if (!mImage->IsComplete() &&
         !ShouldTreatAsCompleteDueToSyncDecode(mImage, mFlags)) {
       mPrepareResult = ImgDrawResult::NOT_READY;
       return false;