[Bokjan]

Fixes #6239.

While a terminal is paused, RenderService defers refreshes until the IntersectionObserver reports it back on screen, which happens after the first frame it is visible in is painted. That's harmless while the canvas still holds the last frame, but it leaves the terminal blank for that frame when the renderer has nothing to show:

• setRenderer while paused: the new renderer starts out empty. Apps that keep the WebGL renderer only for visible terminals, to stay within the WebGL context limit, do this whenever a terminal is shown.
• A sync redraw while paused: Force a sync render after resize occurs #5529 asks for one when the WebGL addon's ResizeObserver resizes the canvas, which clears it, but refreshRows dropped it while paused.

Both now render synchronously while paused: refreshRows with sync runs the paused resize and renders, still marking a full refresh for when rendering resumes, and setRenderer renders right away when paused. Other refreshes still wait for the pause to end.

Verification

• npm run test-unit, with the new RenderService.test.ts: while paused, a new renderer and a sync redraw (after a paused resize) render right away, and other refreshes wait for the observer; while not paused, a new renderer renders on the next frame. The first two fail without the change.
• npm run lint-changes
• The repro from the issue, built against this branch and against master, in Chrome 155 on Windows 11, recording each frame with Page.startScreencast: in 6 switches each, master showed 6 blank frames at pixel ratios of 1 and 1.75, this branch none.

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