For double-precision vertex inputs we need to measure them in dvec4
terms, and for single-precision vertex inputs we need to measure them in
vec4 terms.

For the later case, we use type_size_vec4() function. For the former
case, we had a wrong implementation based on type_size_vec4().

This commit introduces a proper type_size_dvec4() function, that we use
to measure vertex inputs.

Measuring double-precision vertex inputs as dvec4 is required because
ARB_vertex_attrib_64bit states that these uses the same number of
locations than the single-precision version. That is, two consecutives
dvec4 would be located in location "x" and location "x+1", not "x+2".

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>