vec4 attribute or link error for used ones

It is expected that graphics hardware will have a small number of fixed locations for passing vertex
attributes. Therefore, the OpenGL Shading language defines each non-matrix attribute variable as having
space for up to four floating-point values (i.e., a vec4). There is an implementation dependent limit on the
number of attribute variables that can be used and if this is exceeded it will cause a link error. (Declared
attribute variables that are not used do not count against this limit.) A float attribute counts the same
amount against this limit as a vec4, so applications may want to consider packing groups of four unrelated
float attributes together into a vec4 to better utilize the capabilities of the underlying hardware. A matrix
attribute will use up multiple attribute locations. The number of locations used will equal the number of
columns in the matrix.