Previously, we used IMPLEMENTATION_MAX_DRAW_BUFFERS(8) as the array
size of 'previouslyBound' which is used to record whether more than two
different blocks are bound to a single buffer. However, when the
underlying driver is d3d11_1, the max allowed shader storage blocks will
be 64 which is larger than 8. An assert error will be met in
push_back like below:

Test case
'dEQP-GLES31.functional.ssbo.layout.random.all_per_block_buffers.45'..
ERR: push_back(250):    ! Assert failed in push_back
(../../src\common/FixedVector.h:250): mSize < N

So we use 64 as the upper limit of max shader storage blocks.

Bug: angleproject:3447
Change-Id: Ie76bfd15486c1ed02afca4283d6e70bdb4298cb6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1623794
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>