[PATCH 0/2] Support for high DPI outputs via scaling

[John Kåre Alsaker]

I think we should allow fractional scale factors. Clients which only
support integer scale factors should round the scale factor they get to a
whole number.

We should implement this by adding a scale_factor event on wl_output (which
clients can get at any time).
When clients have rendered their contents in the new scale factor they
should call set_scale_factor on wl_surface (which should only work for
windows/top-level surfaces, not subsurfaces).
set_scale_factor should take two arguments, the scale factor of the output
the client used and the rounded scale factor it used for rendering.
The reason the rounded scale factor is sent is so when the compositor
decides to resize the window, it can use the actual scale factor of the
window. The scale factor of the output is send so the compositor knows it
the client rounded the scale factor and can use a different policy of
resizing the window.

I don't think we need a scale priority on wl_outputs, clients should pick
the highest scale factor of the outputs it's on.

Bill Spitzak: This can't be done with the scaler API because it refers to
scaling of subsurfaces which are positioned on pixel boundaries. It would
also be prone to rounded errors resulting in scaling when we don't want any
(which is in most cases).
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