Bug 1804958 - Avoid unnecessary save/restore in ComputeClipExtsInDeviceSpace. r=gfx-reviewers,tnikkel,jrmuizel

I don't expect this to matter for a lot of content, however it avoids an assertion that causes a crashtest to fail.

Differential Revision: https://phabricator.services.mozilla.com/D165291

layout/svg/SVGUtils.cpp

@@ -506,9 +506,10 @@ class MixModeBlender {
     // mode. Clip the source context first, so that we can generate a smaller
     // temporary surface. (Since we will clip this context in
     // SetupContextMatrix, a pair of save/restore is needed.)
-    gfxContextAutoSaveRestore saver(mSourceCtx);
+    gfxContextAutoSaveRestore saver;
 
     if (!mFrame->HasAnyStateBits(NS_FRAME_IS_NONDISPLAY)) {
+      saver.SetContext(mSourceCtx);
       // aFrame has a valid ink overflow rect, so clip to it before calling
       // PushGroup() to minimize the size of the surfaces we'll composite:
       gfxContextMatrixAutoSaveRestore matrixAutoSaveRestore(mSourceCtx);