Metal: Fix MSAA workaround texture being leaked.

Bug: chromium:1070195
Change-Id: Ifb319d567f820bb4d3e9354d24b404bdb9617661
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19281
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/CommandBufferMTL.mm

@@ -990,6 +990,8 @@ namespace dawn_native { namespace metal {
                     CopyIntoTrueResolveTarget(commandContext, trueResolveTextures[i],
                                               trueResolveLevels[i], trueResolveSlices[i],
                                               temporaryResolveTextures[i], width, height);
+                    [temporaryResolveTextures[i] release];
+                    temporaryResolveTextures[i] = nil;
                 }
                 return;
             }