internal/ui: bug fix: do not set the minimum window width for macOS
internal/ui: bug fix: do not set the minimum window width for macOS

Shrinking a window size by a cursor sometimes makes a smaller window
than 126px. To avoid such flaky behavior, do not set the limitation of
the minimum window width.

On Linux/UNIX, there should not be reasons to limit the window width.

Updates #165

hajimehoshi/ebiten,hajimehoshi/ebiten