Fix GamePad window size

[ColinKinloch]

The gamepad window was the wrong size.

I don't know wxWidgets well enough to say why this fix works or whether it will cause issues.

before	after

[zhangbo8418]

The problem is that you can drag and adjust the size of this window with the mouse, but your modification prevents it from being smaller.

[ColinKinloch]

I'll take a closer look. I think the problem is that the window is created at the correct size not accounting for decorations.

This commit fixes that by forcing a resize once the canvas is created. It would be better to the the window to map at the correct size initially and not automatically resize when the canvas is ready.