Wayland: Cache framebuffer size

The calculation of framebuffer size is about to get more complicated
with the introduction of fractional-scale-v1, so only do it in one
place.

This is also a tiny preparation for eventually behaving better towards
wl_egl_window_resize.

src/wl_platform.h

@@ -348,6 +348,7 @@ typedef struct _GLFWscaleWayland
 typedef struct _GLFWwindowWayland
 {
     int                         width, height;
+    int                         fbWidth, fbHeight;
     GLFWbool                    visible;
     GLFWbool                    maximized;
     GLFWbool                    activated;