partially implement external display clipping

we perform external display clipping only on the GL
side (ie: not done on the h/w composer side, which is
harder and would be too risky). in practice this means
that WFD will be clipped properly, while HDMI *may* or
may not depending on how hwc is used.

Bug: 7149437
Change-Id: I92d4d04220db72b6ffb134c7fa7a93af569723a5