On some (primarily older, integrated) drivers, we see significant
time in CPU stalls during updates to the vertex data textures.

As a short term fix, this patch creates an array of vertex data
textures, and rotates which set of them are in use each frame.

There are better long-term options (such as porting the GPU cache
scatter method, or perhaps using UBO/SSBOs here), but this is a
simple workaround for now.

Differential Revision: https://phabricator.services.mozilla.com/D70775

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/b396f2b243fd9433e6038afdf03c905c1e0b3055