Remove didCharSizeChange logic from renderer

The only thing `didCharSizeChange` was used for was to figure out if we
need to refresh the character atlas or not. However, `acquireCharAtlas`
already inspects the atlas owned by this terminal, and if it matches, it
avoids generating a new atlas.

So if `didCharSizeChange` is true and it's not needed, it quickly bails
out as a no-op. But if `didCharSizeChange` is false, and it was needed
(possible given the complexity of testing all of these edge cases), it
could introduce a bug.

I think it makes sense to just get rid of `didCharSizeChange`, and just
always call `acquireCharAtlas`.

src/utils/TestUtils.test.ts

@@ -310,7 +310,7 @@ export class MockRenderer implements IRenderer {
   }
   dimensions: IRenderDimensions;
   setTheme(theme: ITheme): IColorSet { return <IColorSet>{}; }
-  onResize(cols: number, rows: number, didCharSizeChange: boolean): void {}
+  onResize(cols: number, rows: number): void {}
   onCharSizeChanged(): void {}
   onBlur(): void {}
   onFocus(): void {}