…r=gw

As a strating point for the vertex shader, this patch isolates the parts that are common to both shaders: the code that fetches various piece of data, adding a branching point between the text shader and other brushes just after having fetched most of the data. Hopefully we can devise ways to further unify the vertex shaders in followups.

Differential Revision: https://phabricator.services.mozilla.com/D63094

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/26255dd68307e91c80f322ff18233ed22739127d