spice-widget: init egl only after first gl_scanout
spice-widget: init egl only after first gl_scanout

When using GtkDrawingArea and EGL was not used, it was still initialized.
This produced warning messages on systems where EGL is not supported.

Move spice_egl_init from drawing_area_realize to gl_scanout.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=95254

fgouget/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk