Fix GLSL struct with single matrix.

Fix GLSL struct with single matrix.

The parameters of the matrix constructor form an aggregate node that
was passed directly to the constructor of the structure, instead of
creating a one-element aggregate containing the matrix.

https://github.com/mc-imperial/shader-compiler-bugs/issues/74

Bug swiftshader:56

Change-Id: Iff9a2d8dc60d79a0dde28f2aad76407028486ec8
Reviewed-on: https://swiftshader-review.googlesource.com/19308
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>