The glDrawTex() implementation intends to use vertex array pointers,
so set the array buffer to null to prevent it from taking precedence.
Also save/restore projection and modelview matrices.

Bug 23021204

Change-Id: I6b3e59d737a9b75180e6f03e9a686871640f7edd
Reviewed-on: https://swiftshader-review.googlesource.com/3880Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>