internal/ui: bug fix: do not register a framebuffer-size callback on macOS
internal/ui: bug fix: do not register a framebuffer-size callback on macOS

When a decorating state is switched, a framebuffer-size callback was
invoked but with wrong parameters on macOS.

This callback was originally implemented for i3 window manager, and
was not needed for macOS. Then, let's not use this on macOS. Probably
we can also skip this registering on Windows.

Updates #1960
Closes #2257

hajimehoshi/ebiten,hajimehoshi/ebiten