Fix zoom on first click

[MartinBrathen]

This PR fixes #81877

The problem was that fast clicks would zoom on first click. I fixed this by adding the state variable isActive that keeps track of when the preview is active or not.

I also changed the name of a function from changeActive(value) to setActive(value) to make the code more intuitive.

I moved a postMessage call out of the update() function to avoid unnecessary calls.

Please let me know if any of these last two changes are unwanted and I'll revert them.

[MartinBrathen]

@mjbvz I've found a bug where the webviewEditor.onDidChangeViewState eventhandler will not be called in the case where the user performs these actions:

1. Open two images in split view. Image A & B.
2. Click on image A to ensure it has focus.
3. press ctrl | alt + [appropriate number] to give image B focus.
4. Now click on image A

webviewEditor.onDidChangeViewState should get triggered and you should also see how image A gains focus, but this does not happen.

When trying this on stable 1.39.2 the same visual bug appears, where image A does not seem to get focus.

This bug effects this PR and #82074 since both rely on webviewEditor.onDidChangeViewState being triggered.

vscode/extensions/image-preview/src/preview.ts

Line 38 in c2ac36d

webviewEditor.onDidChangeViewState(() => {

[mjbvz]

Thanks. The fix looks good! Can you open a new issue for the webview problem so I can investigate

[MartinBrathen]

Thanks! I'll have it done by tomorrow.