In GLES 3.0, GL_COLOR_ATTACHMENTi enums run all the way up to i=31,
and don't stop at i=15 which the validation was previously checking
against. It's acceptable to use this new enum range also for
EXT_draw_buffers, since an error will still be generated if an enum
is outside the range of maximum supported attachments.

Also, generate INVALID_ENUM when dEQP tests expect it to be generated
for color attachment number that's outside the supported range. This
is not in line with the published 3.0 spec, but that's just an
oversight in the spec document.

Also fix incorrect INVALID_VALUE error in the validation of
renderbufferStorageMultisample to INVALID_OPERATION.

BUG=angleproject:1101
TEST=dEQP-GLES3.functional.negative_api.buffer.* (all pass)

Change-Id: Ib8cf92651d29ef8fe8da0ce4bfa456cbc4d48850
Reviewed-on: https://chromium-review.googlesource.com/332140Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>