Previously if user code called core.getWindowSize() (or
getFramebufferSize) during their update they could get sizes that did
not match engine.gpu_driver.target_desc because the window had changed
size.
Now core.getWindowSize() returns the last size we got from a glfw
callback instead of directly querying the window. So all sizes should
agree during a single frame.
This relies on glfw callbacks only happening when we call
glfw.pollEvents(). This isn't strictly true - a callback could happen
during any glfw call. Might have to revisit this later.
