Bug 1663355 - Always clear and render entire picture cache tiles on Mali-Gxx. r=gw,geckoview-reviewers,agi

On Mali-Gxx there is a driver bug which causes partial updates to offscreen
render targets to fail. This was originally encountered in bug 1558374, where we
thought that the problem was just to do with scissored glClear()s, so we used a
shader to clear the target instead of glClear(). On some sites, however, even
this is not enough, and sometimes renderering to the target fails leaving some
of the previous content in place.

We appear to be able to work around this by ensuring that the entire render
target is cleared, by calling glClear() with the scissor test disabled. This
means that for picture cache tiles we must ensure the entire valid region is
rendered. This patch also reverts the first attempt at a fix from bug 1558374,
as it is no longer necessary since the entire target is being cleared.

Differential Revision: https://phabricator.services.mozilla.com/D90531