macos: Fix glimagesink not always respecting preferred size

Fixes the glimagesink window pretty much never resizing correctly after launching on macOS.

Cocoa version of glwindow only checks the preferred size upon window creation. glimagesink sets the size right before
calling gst_gl_window_show(), which might be way after the window is created in some cases. If the size was set too
late, glimagesink on macOS would remain 320x240 unless manually resized.

This change makes sure to resize the existing window when _show() is called.

Curiously, this has always been an issue, but went from manifesting every once in a while to being almost completely
broken once old event loop workarounds were removed and gst_macos_main() was introduced.