Fixes a GLSL compilation error:
Failed to compile VS: 0:13(43): error: `pos' undeclared
0:13(14): error: operands to arithmetic operators must be numeric
0:13(13): error: operands to arithmetic operators must be numeric
Tested-by: Stefan Dirsch <sndirsch@suse.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>