Fix window flashing background with unspecified size

[m-sasha]

Proposed Changes

The original fix to the window background being visible before the context was to add this in the update callback to AwtWindow:

AwtWindow(
    ...
    update = {
        ...
        if (!it.isDisplayable) {
            it.makeDisplayable()
            it.contentPane.paint(it.graphics)
        }
    }
)

Unfortunately, if the window/dialog size is Unspecified, setSizeSafely calls pack() on the window, to determine its preferred size, making is displayable, and so the code above isn't called.

This PR does the following

• Always call pack() in setSizeSafely to avoid an inconsistent state.
• Call revalidate instead of pack to force composition+layout.
• Improve the check for when the window is shown for the first time.
• Remove window.makeDisplayable() from AwtWindow. Not sure why it was needed.
• Make setSizeSafely a bit more easy to understand.

Testing

Test: Added new unit tests.

Issues Fixed

Fixes: JetBrains/compose-multiplatform#1794

[Walingar]

Please take a look at failing WindowStateTest.set window size by its content when window is on the screen good test case, that is not handled, when size was sepcified and became Unspecified

[m-sasha]

Noticied the changes in setSizeSafely. But setSizeSafely theoretically can be not called. In a case when the window is already this size. So, we won't call pack.

@igordmn setSizeSafely must be called at least once, because the check is

            if (state.size != appliedState.size) {

and the initial value of appliedState.size is null.

[m-sasha]

Changes in the latest commit:

• setSizeSafely now always calls pack(), even if the window is already displayable. That's what was causing the WindowStateTest.set window size by its content when window is on the screen test to fail.
• setSizeSafely is now responsible for updating the composition size after measuring its preferred size, instead of the Window/Dialog update function. It's better because setSizeSafely is the one responsible for "breaking" it.
• The Window/Dialog update function is now responsible only for drawing the first frame.

[Walingar]

LGTM!