scope windows: Fit X axis / Fit all in the menu, and a window that cannot paint says so

The scope menu carried a y fit only: with one shared time span, the x fit *is* the main window's
fit, and the next frame mirrors the result back -- so `Fit X axis` and `Fit all` now sit in the
menu beside the y entries and reset the shared range.

And a blank plot becomes a sentence: a scope window whose GL context failed, or whose area has
painted no frame two seconds after opening, writes the reason into its own title and to stderr
(`[scope] window 'plot 1' is not drawing: GL context failed: ...`). A blank rectangle is
indistinguishable from a failed context or an area that never got a frame, and the report "the
plot is not there, no x axis" is exactly what this answers: if the title stays unchanged, the
area is painting and the question moves to the compositor presenting the second window.

README.md

@@ -146,8 +146,11 @@ mode or the span. **A signal badge dragged onto a scope window's plot lands ther
 drop the main plot takes, into that window's own row set, on the cell the pointer is over — and
 the signal page's *Plot on* buttons list **every window**: one group per window (named, `In
 "plot 2"`), one button per plot of it. A right-click on a scope window's plot opens **its own
-menu** — y axis (auto or a fixed range), fit, remove a trace — acting on that window's rows; a
-detached window's menu carries **Attach back to the main window** beside them.
+menu** — y axis (auto or a fixed range), *Fit X axis*, *Fit all*, remove a trace — acting on that
+window's rows (the x fit resets the **shared** time span, so every view follows); a detached
+window's menu carries **Attach back to the main window** beside them. A window that cannot paint
+says so in its own title after two seconds (and on stderr): a blank plot is indistinguishable from
+a context that failed, and the sentence names which.
 mode or the span.
 
 **The window.** The plot is the window. Under it sits a fixed-height **badge bar** (36 px). Its left