Fix scaling of get_size on Windows

[raphlinus]

Progress toward #26

[dfrg]

Does what it says on the tin!

I was a bit confused until I read #26. I'm not sure how this will play with the scale calculation we do when building the swapchain.

[dfrg]

Works nicely on windows with this fix!

[raphlinus]

Having read that code, I believe that calculation just multiplies by the scale factor, so should give the correct result now. This is all a bit different than the existing druid-shell code, where I think the relevant logic is directly in the WM_SIZE handler. (And, reviewing that code, I think it's less than ideal, because it seems to create a swapchain with fixed size of 1024x768, then do a ResizeBuffers on WM_SIZE. More than once I've felt like I've picked up a rock and watched the bugs scurry around).