Macro expression Overlay.size returns wrong results when using Synchronize Windows

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**Configuration:**
Java 1.8.0_311 (x64)
ImageJ 1.53k
Windows 10.0.19042.1466

**Steps for Reproduction**

1. Open at least 2 multi-slice DICOM series
2. Run _Synchronize Windows_ and click on _Synchronize All_ with default settings
3. Without creating any overlays, use `Overlay.size` in a macro script on any of the open images. The return value is greater `0`.

**Expected bahavior**
The return value of `Overlay.size` should be `0` when used on a synchronized DICOM stack without any previously created overlays.

Update:

I found that this behavior is caused by the _Sync cursor_ setting, i.e. the red cross that is displayed with this setting in all synchronized windows is itself considered an overlay.

@rasband Is this the indented bevavior, this issue can be closed.

Reproduce the report with two multi-slice DICOM series, Synchronize Windows, Synchronize All, and the Sync cursor setting, then run a macro using Overlay.size. Start by tracing how the synchronized red cross is represented as an overlay. Done requires determining whether that cursor should count and resolving the discrepancy between the observed and expected return value.

Written by the indexing model from the issue text.