Given a declaration in the shader as
mat2 M = mat2(vec2(1.0,2.0),
vec2(3.0,4.0) );
vec2 P = M*vec2(5.0,6.0);
How is the matrix filled in ie is it row major so
P=|1 2||5|
|3 4||6|
or is it column major
P=|1 3||5|
|2 4||6|