glsl_types: Rename parameter of glsl_count_attribute_slots

glsl_types: Rename parameter of glsl_count_attribute_slots

glsl_count_attribute_slots takes a parameter to specify whether the
type is being used as a vertex input because on GL double attributes
only take up one slot. Vulkan doesn’t make this distinction so this
patch renames the argument to is_gl_vertex_input in order to make it
more clear that it should always be false on Vulkan.

v2: minor variable renaming (s/member/member_type) (Tapani)

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>