scaled-scene-buffer: reduce unnecessary renderings

scaled-scene-buffer: reduce unnecessary renderings

Prior to this commit, a backing buffer with scale 1 was always created for
a scaled_scene_buffer before showing it, and backing buffers for specific
scales were created on output_enter events.

This commit removes this redundant re-renderings by calling
wlr_scene_buffer_set_dest_size() upon scaled_scene_buffer creation just to
receive output_enter events and delaying the first rendering to the first
output_enter event.

I needed to add font_get_buffer_size() to obtain the size of a font buffer
without actually creating it.