Prevent cairo plugin from overwriting any user-set minimum window size

The cairo plugin presently prevents any use of libdecor_frame_set_min_content_size() because the values set by this function are overwritten by the plugin in ensure_border_surfaces() and replaced by very small values.

This is readily visible adding one libdecor_frame_set_min_content_size() statement to program demo, and seeing that the demo window can be manually resized to much smaller dimensions.

The proposed change is for ensure_border_surfaces() to make sure that the current min-content-size values are as required or larger, and to refrain from changing them if they are already larger.

After this change, the (modified) demo program does obey the user-given minimum window size.