graphic-helpers: fix segfault when medium-size icon file is loaded

graphic-helpers: fix segfault when medium-size icon file is loaded

lab_data_buffer->logical_{width,height} are not the actual size of the
backing buffer, so wlr_buffer.{width,height} should be used when
duplicating it in get_cairo_surface_from_lab_data_buffer().

This mistake caused segfault when an icon file 1~2 times larger than
window.button.{width,height} is loaded, because the buffer for the raw
icon file is reused by setting lab_data_buffer->logical_{width,height}
with the size for display which is different from
wlr_buffer.{width,height} (see buffer_convert_cairo_surface_for_icon()),
and it is duplicated with get_cairo_surface_from_lab_data_buffer() for
hovered/rounded variants of titlebar button.