Keyboard does not scale when display->allocation.width/height == 0

This breaks scaling the on screen keyboard in our QEMU VM:

See

display_set_output_configure_handler:

if (output->allocation.width == 0 &&
                    output->allocation.height == 0)
                        continue;

however e.g. custom modes in QXL have no approprite size information (which can also happen with broken edid):

$ weston-info
…
interface: 'wl_output', version: 3, name: 20
        x: 0, y: 0, scale: 2,
        physical_width: 0 mm, physical_height: 0 mm,
        make: '<Unknown>', model: '<Unknown>',

so this should be handled properly.