There is a slow compiling issue if translate a uniform block with
a large matrix array member to cbuffer, so translate the uniform
block to StructuredBuffer. But pack_matrix pragma directive is
invalid for matrix element of StructuredBuffer, so pack the matrix
into a structure firstly, and then specifies packing alignment for
the matrix in this structure, last the structure is used as
StructuredBuffer element.

Bug: angleproject:4206
Change-Id: I5a2a512e57b37dcdb88c3060d0d078b46f72eb5e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2154525
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>