Emulated points rendering for FL9_3 was getting lucky because the
value of maxDiff just happened to be large enuough to include the
additional pointsprite vertex buffer in the total buffer count used
in IASetVertexBuffers.

maxDiff is calculated while buffers and offsets are being configured.
The pointsprite buffer was not updating maxDiff resulting in an off-by
one calculation which causes TDRs on some Qualcomm chipsets.

This issue was discovered using Cocos2dx.

BUG=angleproject:1176

Change-Id: I3b3af1757e1cef48bbc62e68695a3e239fef77bf
Reviewed-on: https://chromium-review.googlesource.com/305752Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Cooper Partin <coopp@microsoft.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>