Intel driver doesn't like signed integer division.

If you use signed integer division with the Intel HD 4000
driver, it prints a warning which gets turned into an error suggesting
that signed integer division should be avoided. And it's right: even assuming
the shader compiler is sane, signed integer division by a constant
will generate substantially more code than unsigned integer division
by a constant. Since the values here appear to be positive, just cast them
to unsigned int.