When handling the `/set_surface` command, the code would set
plug_page_size to the new value first, and call `sel_plug_pagesize()`
later. The latter then sees the page size is already the same, so it
leaves it unchanged and also does not send the page size to the
OSCSelectObserver object. In practice, this means that only the default
plugin page size from the preferences or set with
`/set_surface/plugin_page_size` take effect and values set with
`/set_surface` are ignored.
Exactly the same thing happens for the send page size.
This code has been like this since it was first introduced in comit
9c0f6ea948 (OSC: Allow set_surface to set send and plugin page sizes.,
2017-06-13)
This commit fixes this by omitting the first assignment.
a9a5787399
