Bug 1969281 - Clear mResolvedDefaultFB on resize. r=ahale a=RyanVM

The size of mDefaultFB and mResolvedDefaultFB diverge on a resize if there is
an existing mResolvedDefaultFB from before the size change. Both need to be
cleared on a resize.

Differential Revision: https://phabricator.services.mozilla.com/D259489

dom/canvas/WebGLContext.cpp

@@ -514,6 +514,7 @@ void WebGLContext::Resize(uvec2 requestedSize) {
   // Kill our current default fb(s), for later lazy allocation.
   mRequestedSize = requestedSize;
   mDefaultFB = nullptr;
+  mResolvedDefaultFB = nullptr;
   mResetLayer = true;  // New size means new Layer.
 }