Fixes black screen issue on EGL+X11 systems.

We weren't setting the backbuffer dimensions on this platform when the window is created.
This required a resize event to first be fired in order to see anything.

So instead do like GLX + X11 platforms do and query the dimensions and set the backbuffer to them.

Should fix issue 7666.
Merge pull request #1372 from Sonicadvance1/fix-egl-blackness

Fixes black screen issue on EGL+X11 systems.