In DEBUG version, when using OpenGL back-ends, ANGLE may meet an
INVALID_OPERATION error from driver when attemping to update an
attribute which is disabled and using a client memory pointer.

This patch fixes this bug by skipping such vertex attributes when
updating them to driver. With this patch the process to update
vertex attributes should be:
(1) For enabled attributes using client memory pointer:
    update by streaming mode in streamAttributes()
(2) For disabled attributes using client memory pointer:
    just label them dirty and skip them
(3) For attributes using buffer objects:
    update with vertexAttrib*Pointer

BUG=angleproject:1942

Change-Id: I57043e5904eb4a342fa22d449d98a957010170d6
Reviewed-on: https://chromium-review.googlesource.com/456747
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>