Fix BidiRenderer.renderText

When creating a Bidi object with a start > 0, Bidi creates a copy of the
text array starting at start, so that it will always consider the text
to render to start from index 0. We need to make sure we use the same
text as Bidi when rendering.

Test: RenderTests.testAllWidgets
Change-Id: I58d2c4b9a03d23b3f1b657f342845e9a42049280