This uses eglPostSubBufferNV to trigger ANGLE to check the window size
and update the size of the swapchain to match, which is recommended
here: https://groups.google.com/d/msg/angleproject/RvyVkjRCQGU/gfKfT64IAgAJ
With the D3D11 backend, using eglPostSubBufferNV with a 0-sized update
region will even skip the Present() call, meaning it won't block for a
vsync period. Hopefully ANGLE will have a less hacky way of doing this
in future. See the relevant ANGLE issue: http://anglebug.com/1438Fixes#3301
James Ross-Gowan
6a3da439cd