Avoid glitch on first frame

[stacyharper]

With the recent wlroots scene-graph api changes, we no longer receive the wl_display@enter output event before the very first frame. Meaning that the first draw max_height is 640. This cause a visual glitch, and a useless loop.

With this change, we use instead the minimum height of the outputs.

[Cloudef]

Does this work with other compositors as well?

[stacyharper]

I haven't tried, but this is an non-greedy solution. It will works great when having only one wl_output. But can still cause edge case issue, when having multiple, with different dimensions. And when no output are enabled (locked screen), we fallback to 640.

It is not a perfect fix.. We would need a way to know which wl_output we will use, before committing the first frame. A kind of get_preferred_output, similar to the get_preferred_scale that we receive early in the flow. I'm still investigating if this is possible somehow. I'll also ask Wayland folks if this has been asked already.