Decode the Bhote Koshi images once for every engine

The event's before and after images went to each engine's display as URLs,
so every engine switch fetched and decoded both again: the strict Nepal event
scenario saw three reads of each image where Cesium's kept providers made one.
The event now decodes the pair once with its own loader and hands the images
to each display, which takes a URL or a decoded image. The pair is dropped
with the rest of the event state on disable, and a failed load is retried on
the next attachment.

The wind engine-switch scenario waited for a diagnostics field the wind layer
lost when it moved onto engine.imagery; it now waits for imageryActive.

Checked in the browser, keyless: the Nepal event, wind and submarine-cable
engine switches pass.

scripts/qa-wind-engine-switch.mjs

@@ -81,7 +81,7 @@ await strict.run({}, async (b) => {
   });
   await p.waitForFunction(
     () =>
-      window.qaWind.getDiagnostics().surface?.ready &&
+      window.qaWind.getDiagnostics().imageryActive &&
       window.qaWind.getDiagnostics().gpu?.ready,
   );
   await p.evaluate(() => window.qaWind.setParams({ inspect: true }));