Lets you query whether this has been marked as resizable via the set_resizable api.
By default, if possible, a this will be created by Cogl as non resizable, but it is not guaranteed that this is always possible for all window systems.
<note>If cogl_onscreen_set_resizable(this, true) has been previously called then this function will return true, but it's possible that the current windowing system being used does not support window resizing (consider fullscreen windows on a phone or a TV). This function is not aware of whether resizing is truly meaningful with your window system, only whether the this has been marked as resizable.</note>
A Onscreen framebuffer
Returns whether this has been marked as resizable or not.