Update ProgramGL to track which the current values of sampler uniforms so
that only the currently applied texture units have to be bound for the
draw call.

BUG=angleproject:882

Change-Id: I280aa106172b13a5fbb31cdefba27b6691c0a0e4
Reviewed-on: https://chromium-review.googlesource.com/264803Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>