Add support for get_render_size()

[abso1utezer0]

The get_display_size() method returns the size of the window, which may differ from the actual render size. It is possible to calculate the render size using aspect ratios, but it would be a lot nicer to be able to get it easily through Dolphin. With the render size, you can align text and other GUI elements to be in the actual game render instead of all over the window.

Here is an example. get_display_size() currently gives the size of the red outline in this image, and the proposed get_render_size() would give the size of the yellow outline in the image.