glsl: fix constructing a vector from a matrix

glsl: fix constructing a vector from a matrix

Without this patch, the following constructs (not an extensive list)
would crash mesa:

- mat2 foo = mat2(1); vec4 bar = vec4(foo);
- mat3 foo = mat3(1); vec4 bar = vec4(foo);
- mat3 foo = mat3(1); ivec4 bar = ivec4(foo);

The first case is explicitely allowed by the GLSL spec, as seen on
page 101 of the GLSL 4.40 spec:

	"vec4(mat2) // the vec4 is column 0 followed by column 1"

The other cases are implicitely allowed also.

The actual changes are quite minimal. We first split each column of
the matrix to a list of vectors and then use them to initialize the
vector. An additional check to make sure that we are not trying to
copy 0 elements of a vector fix the (i)vec4(mat3) case as the last
vector (3rd column) is not needed at all.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>