Implement EGL_LARGEST_PBUFFER.

Bug 23041720

Change-Id: I9f6bed0a3d239a0adde9cc4e90cc368f078f3662
Reviewed-on: https://swiftshader-review.googlesource.com/3902
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>