wayland/output: Report unscaled size even in logical layout mode

In physical layout mode, the size and scale of the `wl_output` matches
the actual monitor:

| Monitor      | `wl_output`  | `GdkMonitor` |
| ------------ | ------------ | ------------ |
| 3840×2160 @1 | 3840×2160 @1 | 3840×2160 @1 |
| 3840×2160 @2 | 3840×2160 @2 | 1920×1080 @2 |

GTK currently does not support `xdg_output`. To estimate the logical
output size for `GdkMonitor`, it divides the output's size by its scale
factor. There might be other legacy clients making the same assumption.

In logical layout mode, mutter currently reports logical geometry for
the `wl_output`s, but this no longer matches the monitors:

| Monitor        | `wl_output`  | `GdkMonitor` |
| -------------- | ------------ | ------------ |
| 3840×2160 @1   | 3840×2160 @1 | 3840×2160 @1 |
| 3840×2160 @2   | 1920×1080 @2 |  960×540  @2 |
| 3840×2160 @1.5 | 2560×1440 @2 | 1280×720  @2 |

This patch changes logical layout mode to multiply the sizes by the
`wl_output`'s scale factor before sending them to the client. Now the
sizes match the physical layout mode again:

| Monitor        | `wl_output`  | `GdkMonitor` |
| -------------- | ------------ | ------------ |
| 3840×2160 @1   | 3840×2160 @1 | 3840×2160 @1 |
| 3840×2160 @2   | 3840×2160 @2 | 1920×1080 @2 |
| 3840×2160 @1.5 | 5120×2880 @2 | 2560×1440 @2 |

Unfortunately, non-integer output scales are not representable in
`wl_output`. Still, I believe these values are better than before, and
the best we can do for clients that do not know about `xdg_output`: The
size of the output will match the size that a buffer for a fullscreen
surface should have at the indicated scale.

Fixes part of https://bugzilla.mozilla.org/show_bug.cgi?id=1534089
GNOME/mutter!510
3 jobs for !510 with wl_output-logical-scale in 11 minutes and 50 seconds
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