JVM Window extended due to JFrame's default BorderLayout Insets

A simple Korge example with jvm target results in the window being larger than the desired height, where the top and bottom contains black borders.

An example:
```kotlin
suspend fun main() = Korge(width = 320, height = 240) {

solidRect(320, 240)
}
```

yields the following:

I believe this is due to the underlying `JFrame`'s default layout manager, which is the `BorderLayout`. During [frame.pack()](https://github.com/korlibs/korge/blob/main/korgw/src/jvmMain/kotlin/com/soywiz/korgw/awt/AwtGameWindow.kt#L263) in `AwtGameWindow`, the preferred size is expanded by the `Insets.top` and `Insets.bottom` of the `BorderLayout`.

Start in korgw/src/jvmMain/kotlin/com/soywiz/korgw/awt/AwtGameWindow.kt around the frame.pack() call, and reproduce the issue with the Kotlin Korge example from the report. Check how the JFrame layout and insets affect the preferred size; done means a 320x240 JVM window no longer gains the reported top and bottom borders.

Written by the indexing model from the issue text.