This fixes an issue with buffers not updating when an output is configured
for a new scale. It also supports windows being on more than 2 outputs at
once and in general simplifies the code.
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.
Implementers can define impl->equal() which compares two
scaled-scene-buffers so that buffers are not allocated for visually
duplicated scaled-scene-buffers.
Currently this mechanism isn't applied for scaled-font-buffers since we
haven't defined impl->equal() for it.
