On Android vector[n] translates to vector[0] likely due to a driver bug.
This resulted in text such as:

    12345678abcdefghijklmnopqrstuvwx

to render as:

    1234123412341234ijklijklijklijkl

It also made graph data flicker.  As a workaround, the shader now does
an if-else on the value of n and uses a constant to load the appropriate
component of the vector.

Bug: angleproject:5690
Change-Id: Idb891e726fafa896a1064ef7b2941b8fd1c41ed1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2725769
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>