Mostly based on the code of pshader_gen_input_modifier_line. The space-adding
behavior of shader_arb_add_src_param was removed because the plurality of
instruction handlers passes an uninitialized buffer in and expects a register
name written to its start, and only map2gl and rcp_rsq use the space-adding
stuff. I'll change rcp_rsq in a later patch anyway. I changed the name to
shader_arb_get_src_param to reflect this behavior.