Don't break subpixel AA when cursor is at the end of longest line

With the Electron upgrade, something changed in the way characters are
rendered/measured, and that was causing subpixel anti-aliasing to stop
working when cursors were at the end of the longest line.

Every cursor has a width that is calculated in the following way:

* If there's a character after the cursor, the width corresponds to
width of such character.
* Otherwise, the width equals to the "base character width" measured on
a dummy line.

In the latter case, even if we were setting the width of the content
container to account for the width of such cursor, some rounding problem
was causing the cursor to be able to escape the container and thus break
subpixel anti-aliasing.

With this commit, instead of rounding the value we assign to the
container width, we will always ceil it. This ensures that cursors are
always strictly contained within the parent element and resolves the
subpixel anti-aliasing issue.

src/text-editor-component.js

@@ -2694,7 +2694,7 @@ class TextEditorComponent {
   }
 
   getContentWidth () {
-    return Math.round(this.getLongestLineWidth() + this.getBaseCharacterWidth())
+    return Math.ceil(this.getLongestLineWidth() + this.getBaseCharacterWidth())
   }
 
   getScrollContainerClientWidthInBaseCharacters () {