glsl: fix matrix stride calculation for std430's row_major matrices with two columns

glsl: fix matrix stride calculation for std430's row_major matrices with two columns

This is the result of applying several rules:

From OpenGL 4.3 spec, section 7.6.2.2 "Standard Uniform Block Layout":

"2. If the member is a two- or four-component vector with components
consuming N basic machine units, the base alignment is 2N or 4N,
respectively."
[...]
"4. If the member is an array of scalars or vectors, the base alignment
and array stride are set to match the base alignment of a single array
element, according to rules (1), (2), and (3), and rounded up to the
base alignment of a vec4."
[...]
"7. If the member is a row-major matrix with C columns and R rows, the
matrix is stored identically to an array of R row vectors with C
components each, according to rule (4)."
[...]
"When using the std430 storage layout, shader storage blocks will be
laid out in buffer storage identically to uniform and shader storage
blocks using the std140 layout, except that the base alignment and
stride of arrays of scalars and vectors in rule 4 and of structures in
rule 9 are not rounded up a multiple of the base alignment of a vec4."

In summary: vec2 has a base alignment of 2*N, a row-major mat2xY is
stored like an array of Y row vectors with 2 components each. Because
of std430 storage layout, the base alignment of the array of vectors
is not rounded up to vec4, so it is still 2*N.

Fixes 15 dEQP tests:

dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_lowp_mat2
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_mediump_mat2
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_highp_mat2
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_lowp_mat2x3
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_mediump_mat2x3
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_highp_mat2x3
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_lowp_mat2x4
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_mediump_mat2x4
dEQP-GLES31.functional.ssbo.layout.single_basic_type.std430.row_major_highp_mat2x4
dEQP-GLES31.functional.ssbo.layout.single_basic_array.std430.row_major_mat2
dEQP-GLES31.functional.ssbo.layout.single_basic_array.std430.row_major_mat2x3
dEQP-GLES31.functional.ssbo.layout.single_basic_array.std430.row_major_mat2x4
dEQP-GLES31.functional.ssbo.layout.instance_array_basic_type.std430.row_major_mat2
dEQP-GLES31.functional.ssbo.layout.instance_array_basic_type.std430.row_major_mat2x3
dEQP-GLES31.functional.ssbo.layout.instance_array_basic_type.std430.row_major_mat2x4

v2:
- Add spec quote in both commit log and code (Timothy)

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com>