Checks for extension support are added to GetBufferPointervOES,
mapBufferOES, unmapBufferOES, mapBufferRangeEXT and
flushMappedBufferRangeEXT.

The GetBufferPointerv function now checks if state is queried from
buffer object zero.

The code is also refactored so that validation happens in separate
validation functions and the implementations are in Context functions.

BUG=angleproject:1101
TEST=dEQP-GLES3.functional.negative_api.state.get_buffer_pointerv
     dEQP-GLES3.functional.*buffer*map* (no regression)

Change-Id: I0f439abd12c92c51324f2e5a31bf621f61534306
Reviewed-on: https://chromium-review.googlesource.com/336164Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>