This saves some time spent in the driver, by making multiple draw
calls happen inside a single RenderPass.

This also makes the ReadPixels impl method non-const. I think in
the future we should avoid making const Impl methods unless they're
totally trivial.

BUG=angleproject:1898

Change-Id: I39172270a2f7dc5c1c2e3d4cc50af3bac8a29fa1
Reviewed-on: https://chromium-review.googlesource.com/672148Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>